Particulate water absorbent and process for production thereof

ABSTRACT

A particulate water absorbing agent of the present invention is a water absorbing agent containing a water absorbing resin as a main component, the particulate water absorbing agent containing a polyvalent metal cation and satisfying: (1) the polyvalent metal cation is contained in an amount between 0.001 wt % and 5 wt % relative to the amount of the water absorbing agent; (2) an absorbency without pressure (CRC) is not less than 28 (g/g) and an absorbency against pressure (AAP 4.83 kPa) is not less than 10 (g/g); (3) the absorbency against pressure and the absorbency without pressure satisfy 77≦AAP (4.83 kPa)+1.8×CRC≦100; and (4) a moisture content of the water absorbing agent is between 5 wt % and 20 wt %. This provides a water absorbing agent which has blocking resistance after moisture absorption, is excellent in stability to shock and suppresses Re-Wet when used in a diaper.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase entry of International ApplicationNo. PCT/JP2010/066957 filed on 29 Sep. 2010, which claims priority under35 U.S.C. §§119(a) and 365(b) to JP Application No. 2009-224756 filed on29 Sep. 2009 and JP Application No. 2010-084530 filed on 31 Mar. 2010.Each of the above recited patent applications are incorporated herein byreference in their entirety.

TECHNICAL FIELD

The present invention relates to a particulate water absorbing agentsuitable for use in sanitary materials such as disposable diapers,sanitary napkins and so-called incontinence pads, and relates to amethod for producing the particulate water absorbing agent.

BACKGROUND ART

An absorbent core constituted by hydrophilic tissue (e.g., pulp) and awater absorbing resin has been in widespread use in sanitary materialssuch as disposable diapers, sanitary napkins and so-called incontinencepads. The absorbent core is used for the purpose of absorbing bodilyfluids.

In recent years, such sanitary materials have been more functionalizedand made thinner. The amount of a water absorbing resin used in eachsanitary product shows an increasing trend, and the ratio of the waterabsorbing resin to the entire absorbent core constituted by the waterabsorbing resin and hydrophilic tissue shows an increasing trend. Thatis, the sanitary products have been reduced in thickness without areduction in their absorbing capacity by increasing the ratio of thewater absorbing resin in the absorbent core by (i) reducing the amountof hydrophilic tissue which is small in bulk density while (ii)increasing the amount of a water absorbing resin which is excellent inwater absorbing property and is large in bulk density.

However, in order to produce sanitary products etc. such as disposablediapers which are to be highly-functionalized and reduced in thickness,it is necessary to incorporate a highly moisture-absorptive waterabsorbing resin into hydrophilic tissue. Depending on operatingenvironment and climate conditions, the water absorbing resin may causeblocking in a storage hopper or in a transport line or may adhere to anapparatus or a pipe etc. This hinders stable production.

In view of this, there have been disclosed techniques of adding aninorganic substance such as amorphous silicon dioxide or kaoline etc. toa water absorbing resin to obtain a water absorbing agent that isexcellent in fluidity after moisture absorption (Patent Literatures 1 to4). However, the techniques of Patent Literatures 1 to 4 have a problemin which water absorbency against pressure of the water absorbing agentis reduced.

Further, there have been disclosed techniques of coating a surface of awater absorbing resin with polysiloxane or a specific surfactant etc.(Patent Literatures 5 to 8). However, the techniques of PatentLiteratures 5 to 8 also have a problem in which, since the waterabsorbing resin is caused to be excessively hydrophobic or its surfacetension is reduced, Re-Wet in disposable diapers is increased. Further,water absorbing resins obtained in Patent Literatures 1 to 15 have, inaddition to the problem of fluidity (blocking resistance) after moistureabsorption, a problem in which its moisture content is generally smalland its particles are low in stability to shock.

Further, according to the technique described in Patent Literature 9,there has been a problem in which a rate of water absorption of aparticulate water absorbing agent decreases because a metallic soap ishydrophobic. Therefore, an absorbing article using such a particulatewater absorbing agent cannot have sufficient water absorbing property.Further, since the metallic soap is handled in a powdery state, therehave been safety and health problems such as deterioration in operatingenvironments due to powder dust or a high risk of dust explosion.Moreover, the techniques disclosed in Patent Literatures 11 and 12 arenot sufficient to obtain a recent particulate water absorbing agenthaving excellent physical properties (e.g., high absorbency againstpressure), and the technique described in Patent Literature 15 has aproblem in which the yield and productivity are low although particlesize is controlled.

In view of such circumstances, there have been disclosed techniques ofadding, in order to improve stability to shock and prevent powder dustetc., a small amount of (several wt % of) water to a surface-crosslinkedwater absorbing resin (Patent Literatures 16 to 20). However, suchimproved techniques also have a problem in which (i) adding water causesadhesive force between particles and thus applies a heavy load on amixing apparatus, and as a result, the mixing apparatus tends to stopmore frequently and (ii) adding a mixing auxiliary agent (e.g.,inorganic salt) causes a reduction in physical properties (e.g.,absorbency against pressure) of the water absorbing resin.

That is, the “stability to shock” and “water absorbing property (CRC,AAP) and fluidity after moisture absorption” are in a trade-offrelationship. According to the conventional techniques, it has beenextremely difficult to improve both of these properties.

The water absorbing agent or the water absorbing resin as has beendescribed is desired to have for example the following properties:excellent free swelling capacity (CRC) and excellent absorbency againstpressure (AAP), high liquid permeability (SFC and GBP etc.), high rateof water absorption (FSR and Vortex), high gel strength, and lowextractable content (Extr). However, since the water absorbingproperties depend on crosslink density, these properties may not bedirectly proportional to each other. For example, if the crosslinkdensity is increased, the gel strength increases but the water absorbingcapacity decreases. In order to suppress such phenomena and to obtain awater absorbing agent that has high absorption capacity and relativelyhigh rate of absorption etc., there has been a method of coatingsurfaces of water absorbing resin particles with a surfactant ornonvolatile carbon hydride. This method improves dispensability of waterabsorbed in the initial stage; however, is not effective enough toimprove the rate of absorption and suction of each of the particles.

In view of such circumstances, there has been proposed a method ofimparting a crosslinked structure not only to inside a water absorbingresin but also to the surface of the water absorbing resin (i.e.,surface crosslinking of a water absorbing resin). For example, therehave been disclosed methods of increasing crosslink density on a surfaceof a water absorbing resin by using an organic surface crosslinkingagent or an inorganic surface crosslinking agent (Patent Literatures 21to 23).

Examples of a known crosslinking agent for use in such methods include:polyhydric alcohols; oxazolidinone; alkylene carbonate; polyhydricglycidyl ethers; haloepoxy compounds; polyhydric aldehydes; polyhydricamines; and polyvalent metal salts. However, using such surfacecrosslinking agents causes a problem in which for example acrosslinkage-forming reaction requires high temperature or takes a longtime and some crosslinking agents will remain unreacted.

In view of circumstances, there has been also known a method of surfacetreatment utilizing a polyvalent metal ionic crosslinking reaction,which can occur at low temperature or room temperature (PatentLiterature 23). However, such a method has a problem (i) in which asurface crosslinking agent made from a polyvalent metal generally haslow physical properties such as absorbency against pressure (AAP) and(ii) the combined use of a polyvalent metal and some other surfacetreatment method causes a reduction in absorbency against pressure(AAP). Further, a surface crosslinking reaction, such as a dehydrationreaction using a polyhydric alcohol serving as a surface crosslinkingagent, requires heat treatment. This excessively reduces moisturecontent of the water absorbing resin and/or causes coloration of thewater absorbing resin. Moreover, there has been a problem in which thereduction in moisture content due to surface crosslinkage reduces powderproperty (shock resistance).

In view of such circumstances, as an alternative to the surfacecrosslinkage using the surface crosslinking agent such as thosedescribed in Patent Literatures 21 to 23 etc., there have been proposedmethods of surface crosslinking using a radical polymerization initiator(Patent Literatures 24, 29, 30) and methods of surface crosslinking forpolymerizing monomers on a surface of a water absorbing resin (PatentLiteratures 25 to 28). Specifically, for example, there has been known amethod of bringing an aqueous solution containing a peroxide radicalinitiator into contact with resin and heating the resin to decompose theradical initiator, thereby introducing crosslinkage into polymer chainsin a shallow surface of the resin (Patent Literature 24).

Further, there has been a method of impregnating a water absorbing resinwith water-soluble unsaturated ethylene monomers and polymerizing thewater-soluble unsaturated ethylene monomers, and heating the waterabsorbing resin to obtain a water absorbing resin improved so that thecrosslink density in shallow surfaces of water absorbing resin particlesis higher than that inside the particles (Patent Literature 25).Furthermore, there have been proposed surface treatment methods each ofwhich is for use in a method of polymerizing monomers on a surface of awater absorbing resin. Each of the surface treatment methods includesadding a radical polymerizable compound to a water absorbing resin andthereafter irradiating the water absorbing resin with activating light(preferably with a ultraviolet ray) (Patent Literatures 26 to 28).Furthermore, there have been proposed surface treatment methods each ofwhich is for use in a method of surface-crosslinking a water absorbingresin with a radical polymerization initiator. Each of the surfacetreatment methods includes irradiation with activating light (preferablywith a ultraviolet ray) (Patent Literatures 29 and 30). Moreover, therehas been proposed a surface treatment method using an ultraviolet ray,which method uses a transition metal such as Ag, Fe, Cr and/or Ce etc.(Patent Literature 31)

According to the methods of Patent Literatures 23 to 31, crosslinkingcan be achieved at low temperatures or room temperature, and a resultingsurface-crosslinked water absorbing resin has high moisture content.However, these methods have a problem in which, generally, theabsorbency against pressure (AAP), in particular absorbency againstpressure under heavy load (AAP 0.7), is difficult to increase.

Further, the surface crosslinking methods described in PatentLiteratures 21 to 30 have a problem in which a surface-crosslinked waterabsorbing resin generally has a low moisture content, and particles ofsuch a water absorbing resin have low stability to shock etc. In orderto improve the stability to shock and to prevent powder dust etc., therehave been proposed techniques of adding about several percentage ofwater to a surface-crosslinked water absorbing resin (Patent Literatures16, 17 and 20). However, such techniques have a problem in which addingwater and its auxiliary agent (e.g., inorganic salt) reduces physicalproperties (e.g., absorbency against pressure) of the water absorbingresin. Further, there has been a problem in which the amount of residualmonomers increases due to surface crosslinking (Patent Literature 32).

Further, conventionally, there have been proposed water absorbing resinsmainly for disposable diapers, in which water absorbing resin manyphysical properties such as absorbency against pressure (AAP), waterabsorbency (CRC) and liquid permeability (GBP, SFC) are controlled(Patent Literatures 17, 20 and 33 to 43). Further, there have been knownwater absorbing resins each having a high moisture content, such asthose described in Patent Literatures 17, 20 and 32. None of these waterabsorbing resins provide good performances, because a large amount ofRe-Wet or leakage etc. occurs when these water absorbing resins are usedin disposable diapers.

CITATION LIST Patent Literatures

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SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide (i) a particulate waterabsorbing agent which is excellent in fluidity after moisture absorptionand is excellent in absorbency against pressure, stability to shock andrate of water absorption and (ii) a method of producing the particulatewater absorbing agent.

Another object of the present invention is to provide (i) a particulatewater absorbing agent which reduces load applied on a mixing apparatuswhen water is added, which contains a certain amount of water, and whosephysical properties are improved and kept and (ii) a method of producingthe particulate water absorbing agent.

A further object of the present invention is to provide a method ofeasily and efficiently producing a particulate water absorbing agentwithout causing (i) deterioration due to powder dust in operatingenvironment and/or (ii) dust explosion due to powder dust.

Providing such particulate water absorbing agents makes it possible toprovide an absorbing article (i) in which the concentration of a waterabsorbing agent used in an absorbent core of a disposable diaper isbetween 20 wt % and 100 wt %, more preferably between 25 wt % and 90 wt%, and particularly preferably between 30 wt % and 80 wt % and (ii)which is excellent in Re-Wet and in urine absorption capacity etc. Inparticular, since the absorbent core is not affected by a damage on thewater absorbing agent even if the water absorbing agent is damaged by anabsorbent core production apparatus, it is possible to provide anabsorbent core which is not affected by apparatuses, causes no leakage,and is excellent in performance. That is, it is possible to achieveexcellent performance in a practical absorbent core.

Solution to Problem

In order to attain the above objects, the present invention provides thefollowing first to fourth water absorbing agents. Further, in order toattain the above objects and further to provide the first to fourthwater absorbing agents, the present invention provides the followingwater absorbing agent production methods 1 to 3.

It should be noted that each of the water absorbing agent productionmethods 1 to 3 of the present invention is a method of producing aparticulate water absorbing agent which (i) contains a water absorbingresin as a main component and (ii) contains a polyvalent metal cationand a certain amount of water. Each of the first to fourth waterabsorbing agents of the present invention can be obtained by for examplethe production methods 1 to 3, and (i) contains a water absorbing resinas a main component and (ii) contains a polyvalent metal cation and acertain amount of water. Further, the first to fourth water absorbingagents and their production methods 1 to 3 have a common object andbring about the same effect.

<First Water Absorbing Agent>

A particulate water absorbing agent (first water absorbing agent) inaccordance with the present invention is a particulate water absorbingagent containing a water absorbing resin as a main component, theparticulate water absorbing agent containing a polyvalent metal (in itssurface) and satisfying the following requirements (1) through (4):

(1) a polyvalent metal cation is contained in an amount between 0.001 wt% and 5 wt % relative to the amount of the particulate water absorbingagent;

(2) an absorbency without pressure (CRC) of the particulate waterabsorbing agent is not less than 28 (g/g) and an absorbency againstpressure (AAP 4.83 kPa) of the particulate water absorbing agent is notless than 10 (g/g);

(3) the absorbency against pressure and the absorbency without pressuresatisfy the inequality: 77≦Absorbency against pressure (AAP 4.83kPa)+1.8×Absorbency without pressure (CRC)≦100; and

(4) a moisture content of the particulate water absorbing agent isbetween 5 wt % and 20 wt %.

The first water absorbing agent is preferably configured such that thepolyvalent metal cation is a metallic soap or a water-soluble polyvalentmetal salt.

<Second Water Absorbing Agent>

A particulate water absorbing agent (second water absorbing agent) inaccordance with the present invention contains: water absorbing resinparticles; a metallic soap (an organic salt of a polyvalent metal);water; and a dispersion stabilizer.

<Third Water Absorbing Agent>

A particulate water absorbing agent (third water absorbing agent) inaccordance with the present invention contains: a water absorbing resin;and a metallic soap (an organic salt of a polyvalent metal), a moisturecontent of the particulate water absorbing agent being between 5 wt %and 20 wt %.

<Fourth Water Absorbing Agent>

A particulate water absorbing agent (fourth water absorbing agent) inaccordance with the present invention is a particulate water absorbingagent containing, as a main component, a surface-treated polyacrylicacid (salt) water absorbing resin, the particulate water absorbing agentsatisfying the following requirements (2), (4) and (5):

(2) an absorbency without pressure (CRC) of the particulate waterabsorbing agent is not less than 28 (g/g) and an absorbency againstpressure (AAP 4.83 kPa) of the particulate water absorbing agent is notless than 10 (g/g);

(4) a moisture content of the particulate water absorbing agent isbetween 5 wt % and 20 wt %; and

(5) a vertical diffusion absorbency under pressure (VDAUP) of theparticulate water absorbing agent is not less than 15 g.

The fourth water absorbing agent preferably contains a water-solublepolyvalent metal salt as a polyvalent metal cation.

Each of the second to fourth water absorbing agents preferably contains0.001 wt % to 5 wt % of polyvalent metal cation(s). Further, each of thesecond to fourth water absorbing agents is preferably configured suchthat the absorbency against pressure and the absorbency without pressuresatisfy the inequality: 77≦Absorbency against pressure (AAP 4.83kPa)+1.8×Absorbency without pressure (CRC)≦100.

<Water Absorbing Agent Production Method 1>

A method of producing a particulate water absorbing agent (waterabsorbing agent production method 1) in accordance with the presentinvention includes mixing an aqueous dispersion containing a metallicsoap (an organic salt of a polyvalent metal) and a dispersion stabilizerwith a water absorbing resin.

<Water Absorbing Agent Production Method 2>

A method of producing a particulate water absorbing agent (waterabsorbing agent production method 2) in accordance with the presentinvention includes: the step of adding a metallic soap (an organic saltof a polyvalent metal) and water to a water absorbing resin; andcontrolling a moisture content of the water absorbing agent to between 5wt % and 20 wt %.

<Water Absorbing Agent Production Method 3>

A method of producing a particulate water absorbing agent (waterabsorbing agent production method 3) in accordance with the presentinvention is a method of producing a particulate water absorbing agentwhich contains a water absorbing resin as a main component, the methodincluding surface-treating the water absorbing resin by a surfacetreatment method including the steps of

(a) mixing an acid radical-containing radical-polymerizable monomer, apolyvalent metal compound and water with the water absorbing resin and

(b) polymerizing the acid radical-containing radical-polymerizablemonomer.

<Relation Between Water Absorbing Agents and Water Absorbing AgentProduction Methods>

A method of producing each of the foregoing water absorbing agents isnot limited to the foregoing production methods. For example, each ofthe foregoing water absorbing agents can be obtained by the foregoingproduction method 1 to 3.

Specifically, the second water absorbing agent can be obtained by forexample the water absorbing agent production method 1.

The third water absorbing agent can be obtained by for example the waterabsorbing agent production method 2.

Further, each of the first and fourth water absorbing agents can beobtained by for example the water absorbing agent production methods 1to 3, which methods preferably include controlling physical propertiesso that a particulate water absorbing agent obtained by mixing apolyvalent metal compound or a metallic soap (an organic salt of apolyvalent metal) satisfies the following requirements (1), (2) and (4).The physical properties can be controlled as appropriate in accordancewith the claims depending from the water absorbing agent productionmethods 1 to 3 or according to this Description etc.

(1) A polyvalent metal cation is contained in an amount between 0.001 wt% and 5 wt % relative to the amount of the particulate water absorbingagent.

(2) An absorbency without pressure (CRC) of the particulate waterabsorbing agent is not less than 28 (g/g) and an absorbency againstpressure (AAP 4.83 kPa) of the particulate water absorbing agent is notless than 10 (g/g).

(4) A moisture content of the particulate water absorbing agent isbetween 5 wt % and 20 wt %.

Advantageous Effects of Invention

According to the present invention, i.e., according to the first tofourth water absorbing agents and their production methods 1 to 3, it ispossible to provide (i) a particulate water absorbing agent which isexcellent in fluidity after moisture absorption (blocking rate aftermoisture absorption), absorbency against pressure (AAP), stability toshock (dusting rate) and rate of water absorption and (ii) a method ofproducing the particulate water absorbing agent. That is, it is possibleto provide an excellent particulate water absorbing agent containing acertain amount of water and a production method thereof, each of whichis capable of improving the performances that are in a trade-offrelationship, i.e., the “stability to shock (dusting rate)” and the“fluidity after moisture absorption (blocking rate after moistureabsorption) and absorbency against pressure (AAP)”.

Further, in a case where a particulate water absorbing agent inaccordance with the present invention is used in a sanitary materialsuch as a disposable diaper, it is possible to prevent the sanitarymaterial from being destroyed due to damage caused to the particulatewater absorbing agent during production of the sanitary material.Accordingly, it is possible to provide an excellent water absorbingproperty.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating elapsed time versus stirring torque afterwater starts being added in Examples 1 and 6 and Comparative Examples 1,3 and 4.

FIG. 2 is a side view schematically illustrating an apparatus formeasuring AAP (absorbency against pressure), which is one of thephysical properties of a water absorbing resin.

DESCRIPTION OF EMBODIMENTS

The following description discusses, in detail, a particulate waterabsorbing agent and a production method thereof in accordance with thepresent invention. Note, however, that the scope of the presentinvention is not limited to the descriptions. Therefore, the presentinvention can be altered as appropriate so as to be implemented,provided that the objects of the present invention are attained.Specifically, the present invention is not limited to the followingembodiments, but may be altered within the scope of the claims. Anembodiment derived from a proper combination of technical meansdisclosed in different embodiments is encompassed in the technical scopeof the present invention. It should be noted that, in this Description,the terms “mass” and “weight” have the same meaning.

[1] Definition of Terms

(1-1) “Water Absorbing Resin”, “Water Absorbing Resin Powder”, “WaterAbsorbing Resin Precursor” and “Water Absorbing Resin Particles”

A “water absorbing resin” as used in the present invention means awater-swelling, water-insoluble gelatinizer in the form of a polymer. Itshould be noted here that the “water-swelling” means that a CRC(centrifugal retention capacity) specified in ERT 441.2-02 is not lessthan 5 [g/g], and the “water-insoluble” means that an Ext (extractablecontent) specified in ERT 470.2-02 is between 0 wt % and 50 wt %.

The water absorbing resin can be designed as appropriate depending onits use, and therefore is not particularly limited in its design. Note,however, that the water absorbing resin is preferably a hydrophiliccrosslinked polymer obtained by polymerizing crosslinking unsaturatedmonomers each having a carboxyl group. Further, the water absorbingresin does not have to consist only of a polymer (i.e., 100 wt %polymer), and therefore can be the one that is surface-crosslinked or acomposition containing an additive etc., provided that the foregoingperformances are achieved.

Further, as used in the present invention, a water absorbing resinobtained by pulverizing the hydrophilic crosslinked polymer to powder,which water absorbing resin has not yet been surface-treated orsurface-crosslinked, is referred to as a “water absorbing resin powder”or a “water absorbing resin precursor” (also referred to as a basepolymer), and a water absorbing resin which has been surface-treated orsurface-crosslinked is referred to as “water absorbing resin particles”.Further, as used in the present invention, water absorbing resins havingdifferent shapes obtained in different steps and a water absorbing resincomposition containing an additive etc. are collectively referred to asa “water absorbing resin”.

(1-2) “Particulate Water Absorbing Agent” and “Hardened Product”

As used in the present invention, the “particulate water absorbingagent” means a gelatinizer for an aqueous liquid, which gelatinizercontains a water absorbing resin as a main component. It should be notedthat the aqueous liquid is not limited to water, and is not limited to aparticular kind provided that the aqueous liquid contains water. Theaqueous liquid may be urine, blood, excrement, waste fluid, moisture andvapor, ice, a mixture of water and an organic solvent and/or aninorganic solvent, rainwater, ground water or the like. Out of these,urine, particularly human urine is more preferred as the aqueous liquid.

The water absorbing resin content in a particulate water absorbing agentin accordance with the present invention is preferably between 60 wt %and 95 wt %, more preferably between 75 wt % and 94 wt %, and furtherpreferably between 85 wt % and 93 wt %, relative to the total weight ofthe particulate water absorbing agent. In addition to the waterabsorbing resin, the particulate water absorbing agent preferablycontains a polyvalent metal compound (described later), a metallic soapand/or water (5 wt % to 20 wt %). The particulate water absorbing agentpreferably further contains a dispersion stabilizer, and furthercontains other component(s) (described later) if needed.

It should be noted that, in the present invention, the one that isobtained by adding a polyvalent metal compound, a polyvalent metalcation (water-soluble polyvalent metal salt), a metallic soap and/orwater, and optionally a dispersion stabilizer to a water absorbing resinis also referred to as a “particulate water absorbing agent”.

In particular, according to the following first to third embodiments andthe water absorbing agent production methods 1 to 3, a water absorbingresin powder to which water only or an aqueous dispersion of a metallicsoap has just been added is so-called less fluid wet powder, because thesurface of the power only is wetted with water (e.g., the foregoing 3 wt% to 25 wt % of water) or is gelatinized with water. Note however that,after a certain period of time, the powder becomes fluid dry powder, asthe water on the surface of the powder spreads and is absorbed into thepowder. According to the present invention, after the aqueous dispersionof the metallic soap is added, the water absorbing resin powder in thewet condition may be allowed to stand for a certain period of time,heated while the moisture content is being maintained, and/or partiallydried, for the purpose of accelerating the spreading of water into thepowder.

It should be noted that the water absorbing resin powder (wet powder),which has entered into the wet condition by addition of water, may causesome particle aggregations as the water spreads from the surface of theparticles into the particles or the water disappears and thus thesurface of the particles dries. In the case where particle aggregationoccurs, although it depends on the degree of aggregation, disintegrationmay be carried out with respect to the water absorbing resin powderhaving some aggregations (aggregates are subjected to an operation ofmechanically crumbling aggregates) before classification. Thedisintegration and classification are optional, and are carried out asappropriate. A usable disintegration apparatus and a usableclassification apparatus are exemplified in the granulation methodsdescribed in U.S. Pat. No. 4,734,478 and U.S. Pat. No. 5,369,148.

Further, the process in which the surfaces of particles are dried bybeing allowed to stand for a certain period of time or dried by heatingand thus the particles lightly aggregate may be referred to as“hardening” or “hardening step” for short. Note here that the fluidityof a hardened dry water absorbing resin powder can be represented as aflow rate specified in ERT 450.2-02. A lightly-aggregated waterabsorbing resin powder is subjected to disintegration and classificationas appropriate to make a water absorbing resin powder (particulate waterabsorbing agent) having preferably a flow rate of for example not morethan 30 [g/sec], more preferably not more than 20 [g/sec], and not morethan 15 [g/sec]. That is, the water absorbing resin powder, to which anaqueous dispersion has been added and which is in the wet condition, maybe allowed to stand for a certain period of time or be heated (or driedby heating), and may further be disintegrated and classified if needed,until the flow rate falls within the above range.

To what degree the particles aggregate depends on the hardening method,and is determined as appropriate. Note however that, generally, thedegree of aggregation is small if the water absorbing resin powder isstirred immediately after water only or an aqueous dispersion of ametallic soap is added, whereas all the particles aggregate into a blockif the water absorbing resin powder is allowed to stand or heatedwithout stirring. It should be noted that the water absorbing resinpowder, which has not yet been hardened but to which an aqueousdispersion of a metallic soap has been added, is in the wet conditionand is poor in fluidity, and the flow rate of such a water absorbingresin powder is generally unmeasurable or 300 seconds or greater. Inthis regard, the flow rate is not particularly limited in the presentinvention.

(1-3) “Particles”

As used in the present invention, the term “particles” means a powderhaving fluidity, preferably a powder whose flow rate (ERT 450.2-02) ismeasurable or a powder that can be classified by sieve classification(ERT 420.2-02). Although the “water absorbing resin powder” described inthe above (1-1) is also regarded as “particles” according to such adefinition, the “water absorbing resin powder” is not referred to as“particles” in the present invention for the purpose of distinguishingwhether surface crosslinkage is present or not.

(1-4) “Polyacrylic Acid (Salt)”

As used in the present invention, the term “polyacrylic acid (salt)”means a polymer that optionally has a graft component and contains, asmain components, recurring units constituted by an acrylic acid and/or asalt thereof (hereinafter referred to as acrylic acid (salt)).Specifically, the “polyacrylic acid (salt)” means a polymer thatcontains, as monomers other than a crosslinking agent, acrylic acids(salts) essentially in an amount between 50 mol % and 100 mol %,preferably in an amount between 70 mol % and 100 mol %, furtherpreferably in an amount between 90 mol % and 100 mol %, and particularlypreferably in an amount of substantially 100 mol %.

(1-5) “EDANA” and “ERT”

The term “EDANA” stands for European Disposables and NonwovensAssociations. The term “ERT” stands for EDANA Recommended Test Methods,which is the European-standard (actually, the world-standard) method ofmeasuring water absorbing resins. In the present invention, unlessotherwise noted, the measurements are carried out in conformity with theoriginal ERT (known document: revised in 2002).

(a) “CRC” (ERT 441.2-02)

The term “CRC” stands for a centrifuge retention capacity, and meansabsorbency without pressure (hereinafter may be referred to as “waterabsorbency”). Specifically, the “CRC” is water absorbency (Unit: [g/g])observed when 0.2 g of a water absorbing resin is allowed to freelyswell in a 0.9 wt % solution of sodium chloride and thereafter water isspun off with use of a centrifugal machine.

(b) “AAP” (ERT 442.2-02)

The term “AAP” stands for absorption against pressure, and means waterabsorbency under pressure. Specifically, the “AAP” is water absorbency(Unit: [g/g]) observed after a water absorbing resin is allowed to swellin a 0.9 wt % solution of sodium chloride under a load of 2.06 kPa (0.3psi) for 1 hour. It should be noted that, although this term ismentioned as “absorption under pressure” in the ERT 442.2-02, the“absorption under pressure” has substantially the same meaning as the“absorption against pressure”. Further, the AAP may be measured under acondition in which the load only is changed to 4.83 kPa (0.7 psi).

(c) “Ext” (ERT 470.2-02)

The term “Ext” stands for Extractables, and means a extractable content(the amount of extractable components). Specifically, the “Ext” is avalue (Unit: wt %) obtained when (i) 1 g of a water absorbing resin isadded to 200 g of a 0.9 wt % solution of sodium chloride and thesolution is stirred at 500 rpm for 16 hours and thereafter (ii) theamount of polymers dissolved is measured by pH titration.

(d) “PSD” (ERT 420.2-02)

The term “PSD” stands for particle size distribution, and means aparticle size distribution measured by sieve classification. It shouldbe noted that weight median particle size (D50) and the particle sizedistribution are measured in the same manner as in the “(1) AverageParticle Diameter and Distribution of Particle Diameter” described inthe pamphlet of International Publication No. WO2004/69915.

(1-6) “Surface Treatment” and “Surface Crosslinking”

As used in the present invention, a surface treatment is a superordinateconcept of a surface crosslinking, and encompasses the surfacecrosslinking. The surface treatment encompasses: addition and mix to thesurface of a water absorbing resin powder (water absorbing resinprecursor, base polymer); coating the surface of the water absorbingresin powder; forming a core-shell structure by polymerizing unsaturatedmonomers etc.; and crosslinking the surface of a water absorbing resinwith a surface crosslinking agent.

For example, the surface treatment means causing an additive to adsorbto the surface of the water absorbing resin powder (physical binding) orcoating the surface of the water absorbing resin powder by adding theadditive to the surface of the water absorbing resin powder. Theadditive is for example inorganic fine particles or a surfactant.Further, the surface treatment means a surface crosslinking, which is asubordinate concept of the surface treatment. That is, the surfacetreatment encompasses (i) radical crosslinking which is carried out byusing a polymerization initiator in a process of polymerizingunsaturated monomers in the surface of the water absorbing resin powder,(ii) physical crosslinking (tangle of polymer chains) which is formed inthe surface of the water absorbing resin powder as a result ofpolymerization of unsaturated monomers, and (iii) covalent bondcrosslinking or ionic bond crosslinking carried out by using a surfacecrosslinking agent.

Further, the surface crosslinking as used in the present inventionmeans, out of the foregoing surface treatments, (i) covalent bondcrosslinking or ionic bond crosslinking carried out by using a surfacecrosslinking agent or (ii) crosslinking between polymer chainsconstituting the surface of the water absorbing resin powder, whichcrosslinking is caused by irradiation with activating light or a radicalinitiator etc.

(1-7) Other

In this Description, the phrase “X to Y”, which is indicative of arange, means that “not less than X but not more than Y”. Further, theterm “t (ton)”, which is a unit of weight, means a “metric ton”.Further, unless otherwise noted, the term “ppm” means “ppm by weight”,and the term “meth(acryl)” means “acryl and/or methacryl”.

[2] Particulate Water Absorbing Agent of the Present Invention (First toFourth Water Absorbing Agents)

In order to attain the foregoing objects, the present invention providesthe following first to fourth water absorbing agents.

(First Water Absorbing Agent)

A particulate water absorbing agent (first water absorbing agent) inaccordance with the present invention has a polyvalent metal on itssurface, and satisfies the following requirements (1) through (4):

(1) a polyvalent metal cation is contained in an amount between 0.001 wt% and 5 wt % relative to the amount of the particulate water absorbingagent;

(2) an absorbency without pressure (CRC) of the particulate waterabsorbing agent is not less than 28 (g/g) and an absorbency againstpressure (AAP 4.83 kPa) of the particulate water absorbing agent is notless than 10 (g/g);

(3) the absorbency against pressure and the absorbency without pressuresatisfy the inequality: 77≦Absorbency against pressure (AAP 4.83kPa)+1.8×Absorbency without pressure (CRC)≦100; and

(4) a moisture content of the particulate water absorbing agent isbetween 5 wt % and 20 wt %.

(Second Water Absorbing Agent)

A particulate water absorbing agent (second water absorbing agent) inaccordance with the present invention contains: water absorbing resinparticles; a metallic soap (an organic salt of a polyvalent metal);water; and a dispersion stabilizer.

(Third Water Absorbing Agent)

A particulate water absorbing agent (third water absorbing agent) inaccordance with the present invention contains a water absorbing resinand a metallic soap (an organic salt of a polyvalent metal), and has amoisture content of between 5 wt % and 20 wt %.

(Fourth Water Absorbing Agent)

A particulate water absorbing agent (fourth water absorbing agent) inaccordance with the present invention is a particulate water absorbingagent which contains, as a main component, a surface-treated polyacrylicacid (salt) water absorbing resin, and satisfies the followingrequirements (2), (4) and (5):

(2) an absorbency without pressure (CRC) of the particulate waterabsorbing agent is not less than 28 (g/g) and an absorbency againstpressure (AAP 4.83 kPa) of the particulate water absorbing agent is notless than 10 (g/g);

(5) a vertical diffusion absorbency under pressure (VDAUP) of theparticulate water absorbing agent is not less than 15 g; and

(4) a moisture content of the particulate water absorbing agent isbetween 5 wt % and 20 wt %.

Each of the second to fourth water absorbing agents preferably contains0.001 wt % to 5 wt % of a polyvalent metal cation(s). Further, each ofthe second to fourth water absorbing agents is preferably configuredsuch that the absorbency against pressure and the absorbency withoutpressure satisfy the inequality: 77≦Absorbency against pressure (AAP4.83 kPa)+1.8×Absorbency without pressure (CRC)≦100.

<Further Preferable Particulate Water Absorbing Agent>

The first particulate water absorbing agent in accordance with thepresent invention is a water absorbing agent in which (i) a polyvalentmetal is present on the surface, (ii) the amount of polyvalent metalcations relative to the amount of the water absorbing agent is between0.001 and 5 parts by weight and the absorbency without pressure (CRC) isnot less than 28 (g/g), and the absorbency against pressure (AAP 4.83kPa) is not less than 10 (g/g), (iii) the absorbency against pressureand the absorbency without pressure satisfy the inequality:77≦Absorbency against pressure (AAP 4.83 kPa)+1.8×Absorbency withoutpressure (CRC)≦100, and (iv) the moisture content is between 5 wt % and20 wt %.

<Polyvalent Metal Cation>

The first particulate water absorbing agent in accordance with thepresent invention is configured such that a polyvalent metal isessentially present on the surface, and the amount of a polyvalent metalcation(s) relative to the amount of the water absorbing agent is between0.001 wt % and 5 wt %, preferably between 0.005 wt % and 3 wt %,particularly preferably between 0.005 wt % and 2 wt %, and mostpreferably between 0.1 wt % and 2 wt %. Controlling the polyvalent metalpresent on the surface of the water absorbing agent so that the amountof the polyvalent metal cation(s) falls within the above range makes itpossible to (i) achieve an optimum powder property (powder friction) andthus improve blendability with pulp etc. when the particulate waterabsorbing agent is used in a disposable diaper etc. and (ii) bring aboutan effect of improving shock resistance. If the amount of the polyvalentmetal cation(s) is less than 0.001 wt %, then it is not possible tobring about the above effect. An amount of the polyvalent metalcation(s) of more than 5 wt % is not preferable, because such an amountcause a reduction in the water absorbing properties (CRC and AAP etc.).Each of the second to fourth water absorbing agents preferably containsalso the foregoing amount of polyvalent metal cation(s) (polyvalentmetal cation(s) derived from a metallic soap or a polyvalent metalcompound). What is important for the first particulate water absorbingagent is that it contains a specific amount of polyvalent metalcation(s), and therefore a counter anion(s) of the cation(s) is notlimited to a particular type. Examples of the counter anion(s) of thepolyvalent metal cation(s) include anionic functional groups of a waterabsorbing resin. In particular, in a case of a polyacrylic acid waterabsorbing resin, the counter anion(s) may be carboxy anion(s).Alternatively, the counter anion(s) may be counter anion(s) of ametallic soap or of a polyvalent metal compound (described later).

It should be noted that the amount of polyvalent metal cation(s) in theparticulate water absorbing agents of the first to fourth embodiments(first to fourth water absorbing agents) of the present invention can befound by fluorescent X-ray. In measuring the amount of polyvalent metalcation(s) by fluorescent X-ray, its measurement conditions etc. can beselected as appropriate. For example, the particulate water absorbingagent is subjected to pre-treatment such as pulverization and/ortrituration. Further, the amount of polyvalent metal cation(s) on thesurface or in a shallow surface of the particulate water absorbing agentcan also be found by grinding and peeling the surfaces of particles witha grinder etc. and then measuring peeled products by fluorescent X-ray.

<CRC>

The CRC of each of the first to fourth particulate water absorbingagents of the present invention is as follows. That is, the CRC of eachof the first and fourth water absorbing agents is essentially not lessthan 28 g/g, whereas the CRC of each of the second and third waterabsorbing agents and the production methods 1 to may be preferably notless than 10 (g/g), more preferably not less than 15 (g/g), not lessthan 18 (g/g), further preferably not less than 20 (g/g), and furthermore preferably not less than 25 (g/g). According to the first to fourthparticulate water absorbing agents each having a CRC of not less than 28g/g, the CRC is particularly preferably not less than 30 (g/g), and mostpreferably not less than 33 (g/g). The CRC is not particularly limitedin its upper limit; however, the upper limit is preferably not more than60 (g/g), more preferably not more than 50 (g/g), further preferably notmore than 45 (g/g), and particularly preferably not more than 40 (g/g).Since the CRC is not less than 10 (g/g), it is possible to achieve waterabsorbing capacity and therefore such a particulate water absorbingagent is suitable for use in sanitary materials such as disposablediapers. Further, since the CRC is not more than 60 (g/g), it ispossible to secure gel strength, and thus possible to obtain highabsorbency against pressure. The CRCs are measured in the mannerdescribed in Examples.

<AAP>

The absorbency against pressure (AAP 4.83 kPa) of each of the first tofourth particulate water absorbing agents of the present invention isnot less than 10 (g/g), more preferably not less than 15 (g/g),particularly preferably not less than 20 (g/g), and most preferably notless than 25 (g/g). The absorbency against pressure (AAP 4.83 kPa) isnot particularly limited in its upper limit; however, in order to bebalanced with the other physical properties, the upper limit is not morethan 40 (g/g), and further not more than about 35 (g/g). The absorbencyagainst pressure is measured in the manner described in Examples.Therefore, the AAPs are controlled to between 10 g/g and 40 g/g, between15 g/g and 35 g/g, between 20 g/g and 30 g/g, between 23 g/g and 30 g/g,and between 25 g/g and 30 g/g. The AAPs can be controlled by surfacecrosslinking or surface treatment. This makes it possible to obtain, byeach of the water absorbing agent production methods 1 to 3, a waterabsorbing agent which has physical properties as high as those ofconventional agents and whose AAP is less reduced (substantially noreduction occurs) even though the water absorbing agent contains apolyvalent metal cation(s).

<Relation Between CRC and AAP>

Further, the CRC and the AAP are related to each other such that the“absorbency against pressure (AAP 4.83 kPa)+1.8× the absorbency withoutpressure (CRC)” is between 77 (g/g) and 100 (g/g), preferably between 77(g/g) and 90 (g/g), and more preferably between 77 (g/g) and 85 (g/g).Controlling the CRC and the AAP so that they satisfy the above conditionallows the particulate water absorbing agent to be excellent in Re-Wetand urine absorption capacity when the particulate water absorbing agentis used in an absorbent core of a disposable diaper, in particular whenused in an amount between 30 wt % and 80 wt %. The CRC and the AAP outof the above range are not preferable, because desired properties of adisposable diaper such as Re-Wet may not be obtained.

For example, it is possible to obtain the first and fourth waterabsorbing agents by controlling, in the water absorbing agent productionmethods 1 to 3, the CRC and the AAP such that a particulate waterabsorbing agent obtained by preferably mixing thereto a polyvalent metalcompound or a metallic soap (an organic salt of a polyvalent metal)satisfies the following requirements (1), (2) and (4):

(1) a polyvalent metal cation is contained in an amount between 0.001 wt% and 5 wt % relative to the amount of the particulate water absorbingagent;

(2) an absorbency without pressure (CRC) of the particulate waterabsorbing agent is not less than 28 (g/g) and an absorbency againstpressure (AAP 4.83 kPa) of the particulate water absorbing agent is notless than 10 (g/g); and

(4) a moisture content of the particulate water absorbing agent isbetween 5 wt % and 20 wt %.

<Moisture Content>

The moisture content of each of the first to fourth particulate waterabsorbing agents of the present invention is between 5 wt % and 20 wt %,preferably between 5 wt % and 18 wt %, more preferably between 6 wt %and 18 wt %, further preferably between 7 wt % and 15 wt %, andparticularly preferably between 8 wt % and 13 wt %. A moisture contentof less than 5% is not preferable, because the particulate waterabsorbing agent becomes difficult to handle due to insufficientstability to shock and further a reduction in fluidity after moistureabsorption. Further, a moisture content of more than 20 wt % is notpreferable, because the water absorbing properties (AAP and CRC etc.)decrease and the fluidity after moisture absorption decreases.

It is preferable that each of the second and third water absorbingagents satisfy the above moisture content.

<Particle Size>

The particle size of each of the first to fourth particulate waterabsorbing agents of the present invention is as follows. That is, eachof the first to fourth particulate water absorbing agents preferablyincludes (i) particles of not less than 106 μm but less than 850 μm indiameter in an amount between 90 wt % and 100 wt % relative to the totalamount of particles and (ii) particles of not less than 300 μm indiameter in an amount of not less than 60 wt % relative to the totalamount of particles.

It is more preferable that the particulate water absorbing agent containparticles of not less than 106 μm but less than 850 μm in diameter inamount of between 95 wt % and 100 wt %, and particularly between 98 wt %and 100 wt %, relative to the total amount of particles.

Further, it is preferable that the particulate water absorbing agentcontain particles of not less than 300 μm in diameter in an amountbetween 65 wt % and 100 wt %, more preferably between 70 wt % and 100 wt%, and particularly preferably between 75 wt % and 100 wt %, relative tothe total amount of particles.

Further, the weight median particle size (D50) of each of the first tofourth particulate water absorbing agents is preferably between 200 μmand 700 μm, further preferably between 300 μm and 600 μm, andparticularly preferably between 400 μm and 500 μm. Further, thelogarithmic standard deviation (σζ), which is an index of uniformity ofparticle size distribution of a particulate water absorbing agent, ispreferably between 0 and 0.40, more preferably between 0 and 0.35, andparticularly preferably between 0 and 0.30.

If a particulate water absorbing agent contains particles of not lessthan 850 μm in diameter in an amount of more than 10 wt % relative tothe total amount of particles, such a particulate water absorbing agentwill cause discomfort in a user, e.g., cause a user a feeling of aforeign body or a feeling of roughness, when used for producing asanitary material such as a disposable diaper. Further, if a particulatewater absorbing agent contains particles of less than 106 μm in diameterin an amount of more than 10 wt % relative to the total amount ofparticles and/or has a logarithmic standard deviation (σζ) of more than0.40, such a particulate water absorbing agent is not preferable becausemany problems occur such as dramatic reductions in absorbency againstpressure and fluidity after moisture absorption, deterioration inoperating environments due to powder dust generated during production ofsanitary materials such as disposable diapers, and an increase insegregation due to wide particle size distribution.

<VDAUP>

The inventors of the present invention found that vertical diffusionabsorbency under pressure (VDAUP), which is a novel parameter indicativeof a physical property, is important for disposable diapers.Specifically, there have been proposed water absorbing resins in whichmany physical properties such as absorbency against pressure (AAP),water absorbency (CRC) and liquid permeability (GBP, SFC) arecontrolled. Under the circumstances, the inventors of the presentinvention found out the vertical diffusion absorbency under pressure(VDAUP), which is closely correlated to physical properties of diapersand with which it is possible to evaluate properties that cannot beevaluated with use of the above parameters.

The vertical diffusion absorbency under pressure VDAUP of the fourthparticulate water absorbing agent of the present invention is preferablynot less than 15 g, more preferably not less than 20 g, furtherpreferably not less than 25 g, further more preferably not less than 30g, particularly preferably not less than 33 g, and most preferably notless than 35 g. In order to be balanced with other physical properties,the upper limit of the VDAUP is not more than 100 g, and furtherpreferably not more than about 80 g. The vertical diffusion absorbencyunder pressure is measured in the manner described in Examples. Inparticular, a VDAUP exceeding the upper limit is not preferable, becausethe CRC dramatically decreases, the desired performance is not obtained,and thus such a particulate water absorbing agent is not suitable forpractical use in a diaper. The above VDAUPs are suitably applicable alsoto the first to third water absorbing agents. A water absorbing agentsatisfying the above VDAUP can be obtained by for example the productionmethods 1 to 3 of the present invention.

The present invention as has been described encompasses the followingpreferred embodiments (A) to (D).

(A) First Embodiment Second Water Absorbing Agent

A water absorbing agent that contains a water absorbing resin, ametallic soap, water, and a dispersion stabilizer.

(B) Second Embodiment Water Absorbing Agent Production Method 1

(i) A water absorbing agent obtained by mixing an aqueous dispersioncontaining a metallic soap and a dispersion stabilizer with a waterabsorbing resin or (ii) a particulate water absorbing agent productionmethod including mixing an aqueous dispersion containing a metallic soapand a dispersion stabilizer with a water absorbing resin.

(C) Third Embodiment Water Absorbing Agent Production Method 2

(i) A particulate water absorbing agent which contains a water absorbingresin and a metallic soap and has a moisture content of between 5 wt %and 20 wt % or (ii) a particulate water absorbing agent productionmethod including: the step of adding a metallic soap and water to awater absorbing resin; and controlling the moisture content of the waterabsorbing agent to between 5 wt % and 20 wt %.

(D) Fourth Embodiment Water Absorbing Agent Production Method 3

A water absorbing agent obtained by a water absorbing resin surfacetreatment method including the steps of: mixing an acidradical-containing radical-polymerizable monomer, a polyvalent metalcompound and water with a water absorbing resin which is a polyacrylicacid (salt) crosslinked polymer; and polymerizing the acidradical-containing radical-polymerizable monomer.

According to the first to third embodiments, it is possible to cause aparticulate water absorbing agent to contain a certain amount of waterand have further improved absorbing properties (AAP and CRC etc.) whilepreventing formation of coarse particles and reducing load applied on amixing apparatus when water is added. That is, the method of the presentinvention is remarkably suitable in view of continuous production,because the load applied on the mixing apparatus when water is added isdramatically reduced. Further, the method of the present invention isremarkably suitable also in view of safety and health, because, sincethe metallic soap is handled in the form of an aqueous dispersion, thereis no problem of powder dust generation.

Note that, according to the fourth embodiment (water absorbing agentproduction method 3), since a surface crosslinking agent thatnecessitates heating at high temperatures is not essential, it ispossible to carry out surface treatment at low temperatures within ashort period of time. Further, the “surface-treated water absorbingresin”, which is obtained by using a combination of two differentsurface treatment methods respectively including polymerizing aradical-polymerizable compound and carrying out ionic crosslinking of apolyvalent metal, has excellent water absorbing properties such asabsorbency against pressure (AAP, VDAUP) as compared to a conventionalwater absorbing resin. Moreover, the surface-treated water absorbingresin obtained by the foregoing method has absorption capacity betterthan a conventional water absorbing resin, when in practical use in anabsorbing article such as a disposable diaper.

The following description discusses, in more detail, the first to fourthembodiments each of which is a preferable embodiment of a particulatewater absorbing agent of the present invention.

[3] Method of Producing Water Absorbing Resin for Use in First to FourthWater Absorbing Agents and Water Absorbing Agent Production Methods 1 to3

It should be noted that the following (3-1) to (3-4) of [3] describe anembodiment of a water absorbing resin powder (water absorbing resinprecursor, base polymer), which is for use in the first to fourth waterabsorbing agents and the production methods 1 to 3.

(3-1) Polymerization Step

This step is a step of polymerizing an aqueous solution containingunsaturated monomers to obtain a hydrous gel crosslinked polymer(hereinafter referred to as a “hydrous gel”).

(a) Unsaturated Monomer (Other than Crosslinking Agent)

Examples of the water absorbing resin of the present invention include:polyacrylic acid (salt) crosslinked polymers; hydrolysates ofstarch-acrylonitrile-grafted polymers; starch-acrylic acid-graftedpolymers; saponified vinyl acetate-acrylic acid ester copolymers;hydrolysates of acrylonitrile copolymers and hydrolysates of acrylamidecopolymers, and crosslinked acrylonitrile copolymers and crosslinkedacrylamide copolymers; denatured crosslinked polyvinyl alcohols eachhaving a carboxyl group; and isobutylene-maleic anhydride crosslinkedcopolymers. These water absorbing resins may be used solely or two ormore water absorbing resins can be used in combination. That is, anunsaturated monomer for use in obtaining the water absorbing resinparticles in accordance with the present invention may be any monomer,provided that it is possible to achieve desired physical properties. Itshould be noted that, in view of physical properties of the waterabsorbing resin particles to be obtained, it is particularly preferableto use a polyacrylic acid (salt) crosslinked polymer (also referred toas a polyacrylic acid (salt) water absorbing resin) as the waterabsorbing resin.

In a case where the polyacrylic acid (salt) crosslinked polymer is to beused, an acrylic acid (salt) (which is an unsaturated monomer) may beused as a main component. In addition, a monomer other than the acrylicacid (salt) (such a monomer is hereinafter referred to as “the othermonomer”) may be used as a copolymerization component. This makes itpossible to impart properties (e.g., anti-bacterial property anddeodorant property) other than the water absorbing property to a finalparticulate water absorbing agent, and also makes it possible to producethe particulate water absorbing agent at lower costs. Such a polyacrylicacid (salt) water absorbing resin optionally has a graft component(e.g., starch, polyvinyl alcohol) in an amount between 0 wt % and 50 wt%, and further preferably in an amount between 0 wt % and 40 wt %. Sucha graft polymer and the foregoing polyacrylic acid (salt) crosslinkedpolymer are collectively referred to as a polyacrylic acid (salt) waterabsorbing resin.

The foregoing other monomer is not particularly limited. Examples of theother monomer include water-soluble unsaturated monomers and hydrophobicunsaturated monomers such as methacrylic acid, maleic acid (anhydride),fumaric acid, crotonic acid, itaconic acid, vinyl sulfonic acid,2-(meth)acrylamide-2-methylpropanesulfonic acid, (meth)acryloxy alkanesulfonic acid and its alkali metal salt and its ammonium salt,N-vinyl-2-pyrrolidone, N-vinylacetamide, (meth)acrylamide, N-isopropyl(meth)acrylamide, N,N-dimethyl (meth)acrylamide, 2-hydroxyethyl(meth)acrylate, methoxy polyethylene glycol (meth)acrylate, polyethyleneglycol (meth)acrylate, isobutylene, and lauryl (meth)acrylate.

Further, the amount of the other monomer to be used is between 0 mol %and 50 mol %, preferably between 0 mol % and 30 mol %, more preferablybetween 0 mol % and 10 mol %, and further preferably between 0 mol % and5 mol %, relative to the total number of moles of unsaturated monomers.In other words, the amount of the acrylic acid (salt) serving as a maincomponent is preferably between 70 mol % and 100 mol %, more preferablybetween 90 mol % and 100 mol %, and further preferably between 95 mol %and 100 mol %. Note however that, in view of water absorbing properties(e.g., AAP) of the particulate water absorbing agent to be obtained, itis most preferable that the amount of the acrylic acid (salt) besubstantially 100 mol %.

In a case where a monomer having an acid radical is used as anunsaturated monomer (including the other monomer), an alkali metal salt,an alkali earth metal salt, and/or an ammonium salt may be used as asalt of the unsaturated monomer. Out of these, in view of properties ofthe particulate water absorbing agent to be obtained, availability inindustry of the unsaturated monomer salt and safety etc., a monovalentsalt is preferable, and a monovalent metal salt is particularlypreferable. Out of these, a sodium salt or a potassium salt ispreferable.

Further, in a case where an acrylic acid (salt) is used as anunsaturated monomer, the ratio of an acrylic acid to an acrylic acidsalt is preferably 0 mol %-50 mol % to 100 mol %-50 mol % (provided thata sum of the acrylic acid and acrylic acid salt is not more than 100 mol%), and is more preferably 10 mol %-40 mol % to 90 mol %-60 mol %. Thatis, the “degree of neutralization”, which is a molar ratio of an acrylicacid salt to the total amount of the acrylic acid and the acrylic acidsalt, is preferably between 50 mol % and 100 mol %, and more preferablybetween 60 mol % and 90 mol %.

The acrylic acid salt is produced for example by (i) neutralizing anacrylic acid in the form of a monomer before polymerization, (ii)neutralizing the acrylic acid in the form of a polymer during or afterpolymerization, or (iii) carrying out a combination of the foregoing (i)and (ii). Further, it is possible to produce an acrylic acid (salt) bymixing an acrylic acid and an acrylic acid salt.

(b) Internal Crosslinking Agent

The water absorbing resin in accordance with the present invention canbe regarded as having an internal crosslinked structure, provided thatthe water absorbing resin is water-swelling and water-insoluble asdescribed in the foregoing (1-1). Therefore, the water absorbing resinmay be the one obtained by self-crosslinking of unsaturated monomerswithout using an internal crosslinking agent. Note, however, that thewater absorbing resin is preferably the one obtained by copolymerizingor reacting unsaturated monomers with an internal crosslinking agent.Examples of the internal crosslinking agent include the one having twoor more unsaturated polymerizable groups or two or more reactive groupsper molecule.

Specific examples of the internal crosslinking agent include:N,N′-methylenebis (meth)acrylamide, (poly)ethyleneglycoldi(meth)acrylate, (poly)propyleneglycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, glycerin tri(meth)acrylate,glycerin acrylate methacrylate, ethyleneoxide-denaturedtrimethylolpropane tri(meth)acrylate, pentaerythritolhexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallylphosphate, triallyl amine, poly(meth)allyloxy alkane,(poly)ethyleneglycol diglycidyl ether, glycerol diglycidyl ether,ethylene glycol, polyethylene glycol, propylene glycol, glycerin,pentaerythritol, ethylenediamine, ethylene carbonate, propylenecarbonate, polyethyleneimine, and glycidyl (meth)acrylate.

These internal crosslinking agents may be used solely or two or moreinternal crosslinking agents may be mixed to be used in combination asappropriate. Further, the internal crosslinking agent may be added to areaction system all at once or may be added in several batches. Further,in view of water absorbing properties etc. of the finished particulatewater absorbing agent, it is preferable to use, when carrying outpolymerization, an internal crosslinking agent having two or moreunsaturated polymerizable groups.

In view of obtaining good physical properties of the water absorbingresin, the amount of the internal crosslinking agent to be used ispreferably between 0.001 mol % and 2 mol %, more preferably between0.005 mol % and 0.5 mol %, further preferably between 0.01 mol % and 0.2mol %, and particularly preferably between 0.03 mol % and 0.15 mol %,relative to the amount of monomers other than the crosslinking agent. Anamount of the internal crosslinking agent of less than 0.001 mol % ormore than 2 mol % is not preferable, because it may be impossible toobtain sufficient water absorbing properties of the water absorbingresin.

In a case where an internal crosslinked structure is to be introducedinto the water absorbing resin with use of the internal crosslinkingagent, the internal crosslinking agent may be added to a reaction systembefore, during or after polymerization of the unsaturated monomers orafter neutralization of the unsaturated monomers.

(c) Polymerization Initiator

A polymerization initiator for use in the polymerization step isselected appropriately depending on the type of polymerization, and isnot particularly limited. Examples of the polymerization initiatorinclude photodegradable polymerization initiators, pyrolyticpolymerization initiators, and redox polymerization initiators.

Examples of the photodegradable polymerization initiators includebenzoin derivatives, benzyl derivatives, acetophenone derivatives,benzophenone derivatives and azo compounds. Examples of the pyrolyticpolymerization initiators include persulfates (sodium persulfate,potassium persulfate, and ammonium persulfate), peroxides (hydrogenperoxide, t-butyl peroxide, methyl-ethyl-ketone peroxide), and azocompounds (2,2′-azobis (2-amidinopropane)dihydrochloride,2,2′-azobis[2-(2-imidazoline 2-yl)propane]dihydrochloride). Examples ofthe redox polymerization initiators include systems each of which is acombination of (i) a reducing compound such as L-ascorbic acid or sodiumhydrogen sulfite and (ii) the foregoing persulfate or peroxide. Further,it is also a preferable embodiment to use the combination of aphotodegradable polymerization initiator and a pyrolytic polymerizationinitiator.

The amount of the polymerization initiator to be used is preferablybetween 0.001 mol % and 2 mol %, and more preferably between 0.01 mol %and 0.1 mol %, relative to the amount of the monomers. An amount of thepolymerization initiator of more than 2 mol % is not preferable, becausethis makes it difficult to control polymerization. Further, an amount ofthe polymerization initiator less than 0.001 mol % is not preferable,because the residual monomer content may increase.

(d) Polymerization Method

According to the foregoing polymerization step, it is possible to carryout bulk polymerization or precipitation polymerization to polymerizethe unsaturated monomers. Note however that, in view of properties ofthe water absorbing resin to be obtained, polymerization controllabilityand water absorbing properties of a hydrous gel etc., it is preferableto employ aqueous polymerization or reversed phase suspensionpolymerization, each of which is carried out by using an aqueoussolution of unsaturated monomers. Note that the unsaturated monomersencompass also the foregoing other monomer and the internal crosslinkingagent.

In a case of preparing the aqueous solution of unsaturated monomers, theconcentration of monomers in the aqueous solution is determined inconsideration of the temperature of the aqueous solution or the type ofmonomers, and is not particularly limited. The concentration ispreferably between 10 wt % and 70 wt %, and more preferably between 20wt % and 60 wt %.

Polymerization of unsaturated monomers is initiated by (i) addition of apolymerization initiator, (ii) irradiation with activating light such asan ultraviolet ray, an electron ray or a gamma ray or (iii) acombination of the foregoing (i) and (ii). A reaction temperature in thepolymerization reaction may be selected as appropriate depending on apolymerization initiator to be used or the type of activating light tobe used, and is not particularly limited. The reaction temperature ispreferably between 15° C. and 130° C., and more preferably between 20°C. and 120° C. A reaction temperature outside the above range is notpreferable, because the residual monomer content in the water absorbingresin to be obtained may increase and/or self-crosslinking reaction mayproceed excessively, and thus the water absorbing properties of thewater absorbing resin may decrease.

It should be noted that the reversed phase suspension polymerization isa method of carrying out polymerization by suspending a monomer aqueoussolution in a hydrophobic organic solvent. Such reversed phasesuspension polymerization is disclosed in for example U.S. Pat. No.4,093,776, U.S. Pat. No. 4,367,323, U.S. Pat. No. 4,446,261, U.S. Pat.No. 4,683,274, and U.S. Pat. No. 5,244,735.

Further, the aqueous polymerization is a method of polymerizing amonomer aqueous solution without using a dispersion solvent. Suchaqueous polymerization is disclosed in for example U.S. Pat. No.4,625,001, U.S. Pat. No. 4,873,299, U.S. Pat. No. 4,286,082, U.S. Pat.No. 4,973,632, U.S. Pat. No. 4,985,518, U.S. Pat. No. 5,124,416, U.S.Pat. No. 5,250,640, U.S. Pat. No. 5,264,495, U.S. Pat. No. 5,145,906,U.S. Pat. No. 5,380,808 and the like, and European Patent No. 0811636,European Patent No. 0955086, European Patent No. 0922717, and the like.Note that, if needed, a solvent other than water may be used incombination. The type etc. of solvent is not particularly limited.

That is, by applying the foregoing unsaturated monomers or thepolymerization initiator etc. to the polymerization method disclosed inthe foregoing patent literatures, it is possible to obtain a waterabsorbing resin in accordance with the present invention.

(3-2) Drying Step

This step is a step of drying a hydrous gel crosslinked polymer (hydrousgel) obtained in the polymerization step. Note that, in a case where thepolymerization step employs aqueous polymerization, pulverization isusually carried out before and/or after the hydrous gel is dried.

The drying step is not particularly limited provided that the dryingstep achieves a desired moisture content, and therefore can employ avariety of methods. Specifically, the drying step may employ drying byheating, hot-air drying, drying under reduced pressure, infrared drying,microwave drying, azeotropic dehydration with a hydrophobic organicsolvent, high humidity drying with use of high-temperature vapor and/orthe like. Out of these, in a case where the hot-air drying is employed,the temperature of the hot air is usually between 60° C. and 250° C.,preferably between 100° C. and 220° C., and more preferably between 120°C. and 200° C. Further, a drying time depends on the surface area of andthe moisture content in the hydrous gel, and the type of dryingapparatus. Therefore, the drying time may be selected as appropriatedfor example within a range of 1 minute to 5 hours so that a desiredmoisture content is achieved.

It should be noted that a hydrous gel obtained by reversed phasesuspension polymerization can be dried in the following manner, withoutcarrying out any pulverization. For example, a hydrocarbon organicsolvent such as a hexane, in which a hydrous gel is dispersed, issubjected to azeotropic dehydration so that the moisture content of thehydrous gel is not more than 40 wt % and preferably not more than 30 wt%. After that, the organic solvent and the hydrous gel are separatedfrom each other by decantation or evaporation to obtain a waterabsorbing resin in accordance with the present invention. Even in thiscase, a drying step may further be carried out if needed. Note that,during the foregoing drying steps, it is possible to carry out surfacecrosslinking simultaneously with drying.

The moisture content [wt %] after the drying step is calculated fromloss on drying (1 g of powder or particles is heated at 180° C. for 3hours). The solid content (100-Moisture content) of a dried resin iscontrolled to be preferably not less than 80 wt %, more preferablybetween 85 wt % and 99 wt %, and further preferably between 90 wt % and98 wt %. In this way, it is possible to a dried polymer.

(3-3) Pulverization Step, Classification Step

This step is a step of pulverizing and/or classifying a dried polymerobtained in the drying step to obtain a water absorbing resin powder. Awater absorbing resin obtained after a pulverization step may bereferred to as a pulverized resin.

It should be noted that, in the case of the reversed phase suspensionpolymerization, the pulverization step is carried out optionally becausethe particle size is controlled during dispersion polymerization. Note,however, that pulverization or disintegration of aggregates (operationof crumbling aggregates) may be carried out if needed. Also in the caseof the aqueous polymerization, it is possible to omit the pulverizationstep after drying, depending on the degree of crush of the gel during orafter polymerization. Note, however, that it is preferable to furthercarry out pulverization and classification.

That is, although a dried polymer obtained in the drying step may bedirectly used as a water absorbing resin powder, the dried polymer ispreferably controlled to have a specific particle size by preferablybeing pulverized and classified so as to obtain a particulate waterabsorbing agent of the present invention. The control of the particlesize can be carried out not only in the pulverization step and theclassification step, but also in the polymerization step, a fine powdercollection step, a granulation step, or the like. In the following, theparticle size is specified by use of a JIS standard sieve (JIS Z8801-1(2000)).

(3-4) Surface Crosslinking Step

As has been described, crosslinking polymerization and drying arecarried out, and if needed, pulverization is carried out to obtain awater absorbing resin powder in accordance with the present invention.It is preferable that the surface of the water absorbing resin powder befurther subjected to crosslinking (secondary crosslinking) to obtainwater absorbing resin particles. This increases the crosslink density inthe shallow surface so as to improve various physical properties of thewater absorbing resin powder.

The following description discusses surface treatment composition(material for surface crosslinking) suitably used in the water absorbingagent production methods 1 and 2. Note that, although the productionmethod 3 (described later) of the present invention employs surfacepolymerization as the surface treatment, the production method 3 mayfurther employ the surface crosslinking, optionally.

According to the water absorbing agent production methods 1 and 2 (andfurther the water absorbing agent production method 3), a surfacecrosslinking agent for use in the surface crosslinking step is notparticularly limited provided that good physical properties of theresulting water absorbing resin particles are achieved. Examples of thesurface crosslinking agent include polyhydric alcohol compounds, epoxycompounds, polyhydric amine compounds, condensates of a polyhydric aminecompound and a haloepoxy compound, oxazoline compounds,monooxazolidinone compounds, dioxazolidinone compounds,polyoxazolidinone compounds, polyvalent metal salts, and alkylenecarbonate compounds. It is preferable that these surface crosslinkingagents be used solely or two or more surface crosslinking agents be usedin combination.

More specifically, it is possible to use the surface crosslinking agentdisclosed in U.S. Pat. No. 6,228,930, U.S. Pat. No. 6,071,976, or U.S.Pat. No. 6,254,990 etc. That is, examples of the surface crosslinkingagent include: polyhydric alcohol compounds such as monoethylene glycol,diethylene glycol, triethylene glycol, tetraethylene glycol,polyethylene glycol, monopropylene glycol, 1,3-propanediol, dipropyleneglycol, 2,3,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin,polyglycerin, 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol,1,5-pentanediol, 1,6-hexanediol, and 1,2-cyclohexane dimethanol; epoxycompounds such as ethylene glycol diglycidyl ether and glycidol;polyhydric amine compounds such as ethylenediamine, diethylenetriamine,triethylenetetramine, tetraethylenepentamine, pent a ethylene hex amine,polyethyleneimine, and polyamide polyamine; haloepoxy compounds such asepichlorohydrin, epibromohydrin and alpha-methylepichlorohydrin;condensates of a polyhydric amine compound and a haloepoxy compound;oxazolidinone compounds such as 2-oxazolidinone; and alkylene carbonatecompounds such as ethylene carbonate.

Further, an ionic bond surface crosslinking agent, a polyvalent metalsalt and/or a polyamine polymer may be used in addition to the foregoingsurface crosslinking agent. Furthermore, an inorganic surfacecrosslinking agent may be used in addition to the foregoing organicsurface crosslinking agent to improve liquid permeability etc. of thewater absorbing agent. Examples of the inorganic surface crosslinkingagent to be used include salts (organic salts or inorganic salts) ofbivalent or multivalent, and preferably trivalent or tetravalent metals;and hydroxides of bivalent or multivalent, and preferably trivalent ortetravalent metals. Examples of usable polyvalent metals includealuminum and zirconium, and aluminum lactate and aluminum sulfate.

The inorganic surface crosslinking agent and the organic surfacecrosslinking agent are used simultaneously with or separately from eachother. Surface crosslinking using a polyvalent metal is exemplified inInternational Publication No. 2007/121037, International Publication No.2008/09843, International Publication No. 2008/09842, U.S. Pat. No.7,157,141, U.S. Pat. No. 6,605,673, U.S. Pat. No. 6,620,889, US PatentApplication Publication No. 2005/0288182, US Patent ApplicationPublication No. 2005/0070671, US Patent Application Publication No.2007/0106013, and US Patent Application Publication No. 2006/0073969,etc.

Further, in addition to the organic surface crosslinking agent, apolyamine polymer, particularly a polyamine polymer having anweight-average molecular weight of 5000 to 1,000,000, may be addedsimultaneously with or separately from the organic surface crosslinkingagent to improve liquid permeability etc of the water absorbing agent.The polyamine polymer to be used is exemplified in for example U.S. Pat.No. 7,098,284, International Publication No. 2006/082188, InternationalPublication No. 2006/082189, International Publication No. 2006/082197,International Publication No. 2006/111402, International Publication No.2006/111403, and International Publication No. 2006/111404 etc.

Out of these surface crosslinking agents, in order to improve variousproperties of water absorbing resin particles as much as possible, it ispreferable to use a covalent bond surface crosslinking agent. Further,in order to prevent a reduction in moisture content during surfacetreatment (to improve stability to shock of water absorbing resinparticles), it is preferable to use an epoxy compound, a haloepoxycompound or an oxazolidinone compound which can react even at lowtemperatures, and further preferable to use at least an epoxy compoundor a haloepoxy compound.

It should be noted that, in a case where the surface crosslinking iscarried out at high temperatures with use of a dehydration-reactivesurface crosslinking agent selected from alkylene carbonates andpolyhydric alcohols, the moisture content is controlled so as to fallwithin the range (described later) by further adding water asappropriate after the surface crosslinking. In a case where a polyhydricalcohol is used as the dehydration-reactive surface crosslinking agent,the polyhydric alcohol is a C₂-C₁₀ polyhydric alcohol, and preferably aC₃-C₈ polyhydric alcohol.

The amount of the surface crosslinking agent to be used depends on (i)the type of surface crosslinking agent to be used and (ii) a combinationof a water absorbing resin precursor and a surface crosslinking agentetc. Note, however, that the amount of the surface crosslinking agent ispreferably between 0.001 and 10 parts by weight, and more preferablybetween 0.01 and 5 parts by weight, relative to 100 parts by weight ofthe water absorbing resin powder.

When carrying out the surface crosslinking, it is preferable to usewater in combination with the surface crosslinking agent. The amount ofwater to be used here depends on the moisture content of the waterabsorbing resin powder to be used. Note however that, usually, theamount of water to be used is between 0.5 and 20 parts by weight, andpreferably between 0.5 and 10 parts by weight, relative to 100 parts byweight of the water absorbing resin powder.

When the surface crosslinking agent or an aqueous solution of thesurface crosslinking agent is to be mixed, a hydrophilic organic solventor a third material may be used as a mixing auxiliary agent.

In a case of using a hydrophilic organic solvent, examples of such ahydrophilic organic solvent include: lower alcohols such as methylalcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butylalcohol, isobutyl alcohol, and t-butyl alcohol; ketones such as acetone;ethers such as dioxane, tetrahydrofuran, and methoxy (poly)ethyleneglycol; amides such as epsilon-caprolactam and N,N-dimethylformamide;sulfoxides such as dimethyl sulfoxide; and polyhydric alcohols such asethylene glycol, diethylene glycol, propylene glycol, triethyleneglycol, tetraethylene glycol, polyethylene glycol, 1,3-propanediol,dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, polypropyleneglycol, glycerin, polyglycerin, 2-butene-1,4-diol, 1,3-butanediol,1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine,triethanolamine, polyoxypropylene, oxyethylene-oxypropylene blockcopolymers, pentaerythritol, and sorbitol.

It should be noted that a polyhydric alcohol is categorized as a surfacecrosslinking agent under the condition where it reacts with a waterabsorbing resin, whereas it is categorized as a hydrophilic organicsolvent under the condition where it does not react with the waterabsorbing resin. Whether the polyhydric alcohol has reacted with thewater absorbing resin or not can be easily determined on the basis ofthe remaining amount of the polyhydric alcohol or an increase in ester(e.g., IR analysis).

The amount of the hydrophilic organic solvent to be used depends on thetype, particle size and moisture content etc. of the water absorbingresin powder. The amount of the hydrophilic organic solvent to be usedis preferably not more than 10 parts by weight, and more preferablybetween 0.1 and 5 parts by weight, relative to 100 parts by weight ofthe solid content of the water absorbing resin powder. Further, aninorganic acid, organic acid and/or polyamino acid etc. shown in thespecification of European Patent No. 0668080 may be caused to exist as athird material. Each of these mixing auxiliary agents may be caused toserve as a surface crosslinking agent, but is preferably the one thatdoes not cause a reduction in water absorbing properties ofsurface-crosslinked water absorbing resin particles. In particular,volatile alcohols each having a boiling point of lower than 150° C. arepreferable, because the volatile alcohols evaporate during the surfacecrosslinking and therefore no residue will remain.

For the purpose of mixing the water absorbing resin powder and thesurface crosslinking agent more evenly, non-crosslinkable, water-solubleinorganic salts (preferably an alkali metal salt, an ammonium salt, ahydroxide of an alkali metal, and ammonia or a hydroxide of ammonia) oran irreducible-alkali-metal-salt pH buffer (preferably bicarbonate,dihydrogen phosphate, hydrogen phosphate etc.) may be caused to coexistwhen the water absorbing resin powder and the surface crosslinking agentare mixed with each other. The amount of a non-crosslinkable,water-soluble inorganic salt or an irreducible-alkali-metal-salt pHbuffer to be used depends on the type and particle size etc. of thewater absorbing resin powder, but is preferably between 0.005 and 10parts by weight, and more preferably between 0.05 and 5 parts by weight,relative to 100 parts by weight of the solid content of the waterabsorbing resin powder.

(How to Add Surface Crosslinking Agent)

The surface crosslinking agent can be added by various methods. Forexample, in a case where the water absorbing resin powder is obtained byaqueous polymerization, it is preferable to (i) if needed, mix thesurface crosslinking agent with water and/or with a hydrophilic organicsolvent in advance during or after the drying step and (ii) mix thesurface crosslinking agent dropwise to the water absorbing resin powder.It is more preferable to spray the surface crosslinking agent to thewater absorbing resin powder. The size of droplets to be sprayed ispreferably between 0.1 μm and 300 μm, and more preferably between 1 μmand 200 μm, in a mean diameter of droplets.

In order to evenly and thoroughly mix the water absorbing resin powder,the surface crosslinking agent, water and/or the hydrophilic organicsolvent, a mixing apparatus to be used to mix these compounds ispreferably the one that has high mixing power. Preferable examples ofsuch a mixing apparatus include: cylindrical mixers, double-wall conicalmixers, high-speed stirring mixers, V-shaped mixers, ribbon mixers,screw type mixers, double-arm kneaders, pulverizing type kneaders,rotating mixers, air mixers, Turbulizer, batch-type Loedige mixers, andcontinuous-type Loedige mixers.

After the surface crosslinking agent and the water absorbing resinpowder are mixed with each other, the mixture is preferably subjected toheat treatment. The heat treatment is carried out under the conditionwhere the temperature of the water absorbing resin powder or thetemperature of a heat medium used in the heat treatment is preferablybetween 60° C. and 250° C., more preferably between 60° C. and 150° C.,and further preferably between 80° C. and 120° C. Further, a heatingtime for the heat treatment is preferably between 1 minute and 2 hours.Examples of a preferable combination of the heating temperature and theheating time include: 180° C. and 0.1 to 1.5 hours; and 100° C. and 0.1to 1 hour.

It should be noted that, in a case where an alkylene carbonate or apolyhydric alcohol is used as the surface crosslinking agent, thetemperature of the water absorbing resin powder or the temperature ofthe heat medium to be used in the heat treatment is preferably between100° C. and 250° C., more preferably between 150° C. and 250° C., andfurther preferably between 170° C. and 210° C. In the case where analkylene carbonate or a polyhydric alcohol is used as the surfacecrosslinking agent and is heated at a temperature falling within theabove range, the moisture content is controlled, after the surfacecrosslinking, to the moisture content (described later) of theparticulate water absorbing agent.

In a case where the water absorbing resin powder is obtained by reversedphase suspension polymerization, it is possible to obtainsurface-crosslinked water absorbing resin particles in the followingmanner. That is, the surface crosslinking agent, preferably a glycidylether compound is dispersed in a hydrophobic organic solvent used in thereversed phase suspension polymerization, for example during azeotropicdehydration and/or after completion of the azeotropic dehydration aftercompletion of the polymerization. That is, the surface crosslinkingagent is dispersed in the hydrophobic organic solvent for example whenthe moisture content of the hydrous gel is not more than 50 wt %,preferably not more than 40 wt %, and more preferably not more than 30wt % (the lower limit is preferably not less than 5 wt %, furtherpreferably not less than 10 wt %, and particularly preferably not lessthan 15 wt %).

(Moisture Content after Surface Crosslinking)

According to these surface crosslinkings, in particular a surfacecrosslinking using a dehydration-reactive surface crosslinking agent(dehydration-reactive crosslinking agent) or a high-temperature surfacecrosslinking (e.g., at a temperature between 150° C. and 250° C.), aresulting water absorbing resin will have excellent physical properties(in particular, AAP 4.83 kPa is excellent). However, the water presentin the water absorbing resin after the drying step and the water addedto the water absorbing resin as a solvent for the crosslinking agentwill be mostly removed. Accordingly, the moisture content of the waterabsorbing resin after the surface crosslinking reaction is usually aslow as 0 wt % to 5 wt %, 0 wt % to 3 wt %, or 0 wt % to 1 wt %. That is,(i) excellent physical properties that generally necessitatehigh-temperature surface treatment and (ii) high moisture content are ina trade-off relationship.

For this reason, although there has been proposed a conventionaltechnique of adding water after the high-temperature treatment of thesurface, it has been difficult to produce a water absorbing agent stablyin production processes.

That is, such a water absorbing resin having a low moisture content hasa problem of stability to shock. In view of this, there have beenproposed techniques of adding an approximately several percentage ofwater to a surface-crosslinked water absorbing resin (Patent Literatures15 to 20). However, such techniques have (i) a problem in whichparticles stick to one another when water is added, and as the amount ofwater added is increased, a higher load is applied on a mixer andthereby the mixer often stops operating and (ii) a problem in whichadding a mixing auxiliary agent (e.g., inorganic salt) causes areduction in physical properties (e.g., absorbency against pressure) ofthe water absorbing resin. In order to solve such problems, a productionmethod of the present invention includes adding water in the form of anaqueous dispersion of a metallic soap so that the water absorbing resinhave a predetermined moisture content (between 5 wt % and 20 wt %). Assuch, the present invention is preferable because, even if water isadded to a surface-crosslinked water absorbing resin having a lowmoisture content (between 0 wt % and 5 wt %, between 0 wt % and 3 wt %,between 0 wt % and 1 wt %), the physical properties after surfacecrosslinking are not reduced substantially.

[4] Preferable Physical Properties of Water Absorbing Resin Particles inWater Absorbing Agent Production Methods 1 to 3

It is preferable to obtain the first to fourth water absorbing agents ofthe present invention by the water absorbing agent production methods 1to 3 using water absorbing resin particles having the following physicalproperties, in particular by the production methods 1 and 2 in which thewater absorbing resin particles are surface-crosslinked so as to havethe following physical properties.

(4-1) Particle Size

Water absorbing resin particles in accordance with the presentinvention, which are obtained by being subjected to a surfacecrosslinking as appropriate, are controlled to have a certain particlesize in order to secure fluidity after moisture absorption and tosuppress a reduction in water absorbing properties and a reduction influidity after moisture absorption caused by mechanical shock.Specifically, the water absorbing resin particles are controlled to havea weight median particle size (D50) of preferably between 200 μm and 700μm, more preferably between 300 μm and 600 μm, and further preferablybetween 400 μm and 500 μm.

Further, the particulate water absorbing agent of the present inventionpreferably contains particles of not less than 106 μm but less than 850μm in diameter in an amount between 90 wt % and 100 wt %, morepreferably between 95 wt % and 100 wt %, and further preferably between98 wt % and 100 wt %, relative to the total amount of water absorbingresin particles contained in the particulate water absorbing agent.Furthermore, the particulate water absorbing agent preferably containsparticles of not less than 300 μm in diameter in an amount between 60 wt% and 100 wt %, more preferably between 65 wt % and 100 wt %, furtherpreferably between 70 wt % and 100 wt %, and particularly preferablybetween 75 wt % and 100 wt %. Moreover, as to particle size distributionof the water absorbing resin particles, logarithmic standard deviation(σζ), which is an index indicative of uniformity, is preferably between0 and 0.40, more preferably between 0 and 0.35, and most preferablybetween 0 and 0.30.

Therefore, if the amount of water absorbing resin particles of not lessthan 850 μm in diameter is more than 10 wt % relative to the totalamount of the water absorbing resin particles contained in theparticulate water absorbing agent, such a particulate water absorbingagent is not preferable because it will cause discomfort in a user,e.g., cause a user a feeling of a foreign body or a feeling ofroughness, when used in a sanitary material such as a disposable diaper.Further, if the amount of water absorbing particles of less than 106 μmin diameter is more than 10 wt % relative to the total amount of waterabsorbing resin particles contained in the particulate water absorbingagent and/or the logarithmic standard deviation (σζ) is more than 0.40,such a particulate water absorbing agent is not preferable because manyproblems occur such as a dramatic reduction in absorbency againstpressure and a reduction in fluidity after moisture absorption,deterioration in operating environments due to powder dust generation,and an increase in segregation due to wide particle size distribution.

It should be noted that the weight median particle size (D50) and theamount of particles having a certain particle size can be controlled asappropriate by controlling the particle size of a water absorbing resinwhich has not yet been surface-treated during the polymerization step(particularly reversed phase suspension polymerization), thepulverization and classification steps, the granulation step or the finepowder collection step etc. A surface-treated water absorbing resin mayalso be subjected to a disintegration step (step of crumbling aggregatesformed during surface treatment), a particle sizing step (classifyingwith use of a sieve or blending particle components having differentparticle sizes), granulation step and/or fine powder collection stepetc. as appropriate so as to have a predetermined amount of particleshaving a certain particle size. The control to obtain a predeterminedparticle size is for example carried out by any of the steps describedin US Patent Application Publication No. 2004/181031, US PatentApplication Publication No. 2004/242761, and US Patent ApplicationPublication No. 2006/247351 etc.

(4-2) CRC

Water absorbing resin particles in accordance with the present inventionhave a centrifugal retention capacity (CRC) of preferably not less than10 [g/g], more preferably not less than 15 [g/g], further preferably notless than 25 [g/g], particularly preferably not less than 28 [g/g], moreparticularly preferably not less than 30 [g/g], and most preferably notless than 33 [g/g]. The upper limit of the CRC is not particularlylimited, but is preferably not more than 60 [g/g], more preferably notmore than 50 [g/g], further preferably not more than 45 [g/g], andparticularly preferably not more than 40 [g/g]. If the centrifugalretention capacity (CRC) is less than 10 [g/g], water absorbing capacityis too small and such water absorbing resin particles are not suitablefor use in a sanitary material such as a disposable diaper. On the otherhand, if the centrifugal retention capacity (CRC) is more than 60 [g/g],it may be impossible to obtain a particulate water absorbing agent thatis excellent in rate of liquid absorption into an absorbent core whenthe particulate water absorbing agent is used in the absorbent core.Note that, according to the present invention, in a case where the firstor fourth water absorbing agent whose CRC is also not less than 28 [g/g]is to be obtained, usually, a water absorbing resin also having a CRC ofnot less than 28 [g/g] is selected as appropriate.

(4-3) AAP 4.83 kPa

Water absorbing resin particles in accordance with the present inventionhave an absorbency under a pressure (load) of 4.83 kPa (AAP 4.83 kPa) ofnot less than 10 [g/g], preferably not less than 15 [g/g], morepreferably not less than 18 [g/g], further preferably not less than 20[g/g], further preferably not less than 25 [g/g], particularlypreferably not less than 28 [g/g], and most preferably not less than 30[g/g].

Such an AAP can be achieved by for example surface-crosslinking thewater absorbing resin particles having the foregoing particle size, inparticular, by surface-crosslinking the water absorbing resin particlesso as to control the CRC to a predetermined CRC. Note, however, thatthis does not imply any limitation.

(4-4) Moisture Content

The moisture content of a water absorbing resin (water absorbing resinprecursor, base polymer, surface-crosslinked water absorbing resinparticles) in accordance with the present invention is not particularlylimited. Note however that, in order to obtain a powder having fluidityeven at room temperature so as to obtain a particulate water absorbingagent of the present invention, it is preferable to control the moisturecontent. Accordingly, the moisture content of the water absorbing resinparticles is more than 0 wt % but not more than 20 wt %, preferably morethan 0 wt % but not more than 10 wt %, more preferably between 0.1 wt %and 7 wt %, and particularly preferably between 0.1 wt % and 5 wt %. Themoisture content of the surface-crosslinked water absorbing resinparticles is between 0 wt % and 5 wt %, more preferably between 0 wt %and 3 wt %, and particularly preferably between 0 wt % and 1 wt %.

It should be noted that the moisture content of a particulate waterabsorbing agent (third water absorbing agent) of the present inventionobtained by the water absorbing agent production method 1 or 2, i.e.,the moisture content of a particulate water absorbing agent obtained byadding a metallic soap to water absorbing resin particles, falls withina predetermined range. That is, the moisture content of the particulatewater absorbing agent is between 5 wt % and 20 wt %, preferably between5 wt % and 18 wt %, and more preferably between 6 wt % and 18 wt % (thisis described later).

Further, the moisture content of a particulate water absorbing agent(fourth water absorbing agent) of the present invention obtained by thewater absorbing agent production method 3 also falls within apredetermined range. That is, the moisture content of the particulatewater absorbing agent is between 5 wt % and 20 wt %, preferably between5 wt % and 18 wt %, and more preferably between 6 wt % and 18 wt %.

How to control the moisture content of a particulate water absorbingagent of the present invention so that the moisture content falls withina predetermined range is also described later.

[5] Particulate Water Absorbing Agent Production Methods Related toFirst to Fourth Water Absorbing Agents

(a) Production Methods

(Water Absorbing Agent Production Method 1)

A method of producing a particulate water absorbing agent (waterabsorbing agent production method 1) of the present invention includesmixing an aqueous dispersion containing a metallic soap (an organic saltof a polyvalent metal) and a dispersion stabilizer with a waterabsorbing resin.

(Water Absorbing Agent Production Method 2)

A method of producing a particulate water absorbing agent (waterabsorbing agent production method 2) of the present invention includesthe steps of adding a metallic soap (an organic salt of a polyvalentmetal) and water to a water absorbing resin and controlling a moisturecontent of the particulate water absorbing agent to between 5 wt % and20 wt %.

(Second Water Absorbing Agent)

A particulate water absorbing agent (second water absorbing agent) ofthe present invention includes: water absorbing resin particles; ametallic soap (an organic salt of a polyvalent metal); water; and adispersion stabilizer.

(Third Water Absorbing Agent)

A particulate water absorbing agent (third water absorbing agent) of thepresent invention include: a water absorbing resin and a metallic soap(an organic salt of a polyvalent metal), and has a moisture content ofbetween 5 wt % and 20 wt %.

A method of producing each of the water absorbing agents is not limitedto the above production methods. For example, each of the waterabsorbing agents can be obtained by the production methods 1 to 3. Thatis, the second water absorbing agent can be obtained by for example thewater absorbing agent production method 1. The third water absorbingagent can be obtained by for example the water absorbing agentproduction method 2.

Further, each of the first and fourth water absorbing agents can beobtained by for example the water absorbing agent production methods 1to 3, each of which methods preferably includes carrying out control sothat a particulate water absorbing agent obtained by mixing a polyvalentmetal compound or a metallic soap (an organic salt of a polyvalentmetal) with a water absorbing resin satisfies the following requirements(1), (2) and (4):

(1) a polyvalent metal cation is contained in an amount between 0.001 wt% and 5 wt % relative to the amount of the particulate water absorbingagent;

(2) an absorbency without pressure (CRC) of the particulate waterabsorbing agent is not less than 28 (g/g) and an absorbency againstpressure (AAP 4.83 kPa) of the particulate water absorbing agent is notless than 10 (g/g); and

(4) a moisture content of the particulate water absorbing agent isbetween 5 wt % and 20 wt %.

A method of producing a particulate water absorbing agent (productionmethod 3) in accordance with the present invention includes controllingthe moisture content of a particulate water absorbing agent, which isobtained by adding a metallic soap and water to the water absorbingresin (water absorbing resin powder or water absorbing resin particles),so that the moisture content is between 5 wt % and 20 wt %.

A method of adding the metallic soap and water is not particularlylimited. For example, it is possible to employ any of the followingproduction methods a to g.

It should be noted that a water absorbing resin contained in aparticulate water absorbing agent of the present invention is notparticularly limited, and may either be a water absorbing resin powderthat has not been surface-crosslinked or surface-crosslinked waterabsorbing resin particles.

(Production Method a)

A metallic soap is added to a water absorbing resin powder to obtain amixture. After that, the mixture is surface-crosslinked to obtain waterabsorbing resin particles. After that, water is added to obtain aparticulate water absorbing agent of the present invention.

(Production Method b)

A water absorbing resin powder is surface-crosslinked to obtain waterabsorbing resin particles. After that, a metallic soap is added. Afterthat, water is added to obtain a particulate water absorbing agent ofthe present invention.

(Production Method c)

A water absorbing resin powder is surface-crosslinked to obtain waterabsorbing resin particles. After that, an aqueous dispersion of ametallic soap is added to obtain a particulate water absorbing agent ofthe present invention.

(Production Method d)

A water absorbing resin powder is surface-crosslinked to obtain waterabsorbing resin particles. After that, water is added. After that, ametallic soap is added to obtain a particulate water absorbing agent ofthe present invention.

(Production Method e)

When a water absorbing resin powder is surface-crosslinked, a surfacecrosslinking agent and a metallic soap are simultaneously added toobtain a particulate water absorbing agent of the present invention.

(Production Method f)

A metallic soap in a powdery state or an aqueous dispersion of ametallic soap is added to a hydrous gel that has not yet been dried(i.e., a hydrous gel having a moisture content of between 10 wt % and 80wt %, preferably between 20 wt % and 70 wt %). After that, a resultanthydrous gel is dried to obtain a particulate water absorbing agent ofthe present invention.

(Production Method g)

The same operations as in the production method f are repeated exceptthat, after the metallic soap in the powdery state or the aqueousdispersion of the metallic soap is added to the hydrous gel that has notyet been dried and then the resultant hydrous gel is dried, a metallicsoap in a powdery state or an aqueous dispersion of a metallic soap isfurther added to a dried gel or added during or after surface treatment.In this way, a particulate water absorbing agent of the presentinvention is produced.

According to the water absorbing agent production methods 1 and 2, it ispossible to obtain the third particulate water absorbing agent of thepresent invention by controlling, after adding a metallic soap by theproduction method a, b, c, d, e, f or g, the moisture content so thatthe moisture content of a final product will fall within the foregoingrange, i.e., between 5 wt % and 20 wt %, preferably between 5 wt % and18 wt %, and more preferably between 6 wt % and 18 wt %.

Instead of the foregoing production methods a to g, a metallic soap canbe added by for example (i) a dry blending method by which to directlymix a metallic soap in a powdery state to a water absorbing resin or(ii) a method of dispersing a metallic soap in a powdery state in waterwith use of a dispersion stabilizer to obtain an aqueous dispersion ofthe metallic soap and then mixing the aqueous dispersion of the metallicsoap with a water absorbing resin. Out of these, in the presentinvention, the method of adding the aqueous dispersion of the metallicsoap is more preferable than the dry blending method, because the dryblending method entails a risk of deterioration in operatingenvironments due to powder dust and a risk of dust explosion due topowder dust.

In the production methods a to g, in a case where a metallic soap isadded in the form of an aqueous dispersion, the metallic soap and waterare added simultaneously. Accordingly, the metallic soap efficientlyadhere to the surface of a water absorbing resin, and particles of thewater absorbing resin becomes less adhesive to each other. This attainsthe objects of the present invention to a greater extent.

The aqueous dispersion of the metallic soap to be added preferablycontains water and a dispersion stabilizer (preferably a surfactant) inamounts described later. Adding such an aqueous dispersion will providea water absorbing agent that contains the water and the dispersionstabilizer in amounts falling within the preferable ranges. Further,before or after the aqueous dispersion of the metallic soap is added,the water and/or the dispersion stabilizer in a water absorbing resinmay be dried as appropriate, or water or a dispersion stabilizer mayfurther be added to the water absorbing resin. It is preferable that thewater and the dispersion stabilizer (surfactant) which were addedsimultaneously with the metallic soap remain contained in a waterabsorbing agent for use, provided that the amount thereof falls withinthe range described later.

Further, in an aqueous dispersion containing a metallic soap and adispersion stabilizer for use in the present invention, an additive(described later, e.g., a chelating agent, a deodorizer, ananti-coloring agent) serving as the other component may be dissolved ordispersed so as to further modify the water absorbing agent, providedthat the amount of the additive falls within the range described later.Furthermore, granulation and/or suppression of fine powder (the finepowder is for example made up of water absorbing resin particles of lessthan 150 μm in diameter) may be carried out by adding an aqueousdispersion of a metallic soap.

The granulation may be carried out by heating the foregoing mixture,pulverizing the mixture if needed, and essentially classifying themixture. Further, the granulation is carried out such that the weightmedian particle size of a particulate water absorbing agent becomespreferably 1.01 to 10 times, more preferably 1.02 to 2 times, andparticularly preferably 1.05 to 1.5 times the weight median particlesize before the aqueous dispersion of the metallic soap is added. Itshould be noted that the granulation is carried out by binding aplurality of water absorbing resins at their surfaces. The granulationis suitably applicable to the water absorbing agent production method 3and to the first to fourth water absorbing agents.

The amounts of a metallic soap, water and a dispersion stabilizercontained in each of the second and third (and further the first andfourth) particulate water absorbing agents and/or the amounts of ametallic soap, water and a dispersion stabilizer to be added to a waterabsorbing resin are determined relatively to 100 parts by weight of thewater absorbing resin. The amount of a metallic soap is preferablybetween 0.001 and 5 parts by weight, more preferably between 0.001 and 3parts by weight, and further preferably between 0.01 and 3 parts byweight. An amount of a metallic soap contained and/or an amount of ametallic soap to be added falling within the above range are/ispreferable, because fluidity after moisture absorption is dramaticallyimproved and load to be applied on a mixer when water is added issignificantly reduced. The amount of water is preferably between 3 and25 parts by weight, more preferably between 3 and 20 parts by weight,and further preferably between 5 and 10 pats by weight. Further, theamount of a dispersion stabilizer to be added simultaneously orseparately is preferably between 0.0001 and 1 part by weight, morepreferably between 0.001 and 1 part by weight, and particularlypreferably between 0.002 and 0.5 part by weight. An amount of adispersion stabilizer contained and/or an amount of a dispersionstabilizer to be added falling within the above range are/is preferable,because the rate of water absorption is dramatically improved.

Accordingly, in a case where the second and third (and further first orfourth) particulate water absorbing agents are to be obtained by addinga metallic soap and water to a water absorbing resin, it is preferablethat the metallic soap be added in an amount between 0.001 and 5 partsby weight and the water be added in an amount between 3 and 25 parts byweight, relative to 100 parts by weight of the water absorbing resin.

According to the present invention, in a case where a metallic soap isadded in the form of an aqueous dispersion, (i) the concentration of themetallic soap in the aqueous dispersion is preferably between 1 wt % and90 wt %, more preferably between 1 wt % and 60 wt %, and furtherpreferably between 1 wt % and 40 wt % and (ii) the concentration of adispersion stabilizer in the aqueous dispersion is preferably more than0 wt % but not more than 10 wt %, more preferably more than 0 wt % butnot more than 5 wt %, and further preferably more than 0 wt % but notmore than 3 wt %. Further, the amounts of the metallic soap, dispersionstabilizer and water contained in the aqueous dispersion are controlledto fall within the above range relative to 100 parts by weight of thewater absorbing resin. By adding the foregoing amount(s) of (a) waterand a metallic soap or (b) an aqueous dispersion of a metallic soap asabove and further controlling the moisture content so as to achieve themoisture content described later, stability to shock and dusting rateare dramatically improved.

According to the production method 1 (and further the production method2) of the present invention, stirring torque (defined in the measurementmethod in Examples) after an aqueous dispersion is added is low.Accordingly, no excessive load is applied during stirring when water isadded, and water absorbing resin particles are not destroyed. Thisachieves excellent physical properties and prevents an increase ingeneration of powder dust. The stirring torque is not more than 1.5[N·m], preferably not more than 1.0 [N·m], and further preferably notmore than 0.70 [N·m].

Further, according to the production method 1 (and further theproduction method 2) of the present invention using an aqueousdispersion of a metallic soap, few coarse particles (aggregatedparticles, excessively-aggregated particles) are generated even if wateris added to a water absorbing resin powder. Therefore, a resulting waterabsorbing resin contains few coarse particles. Specifically, the amountof coarse particles contained in the water absorbing resin is between 0wt % and not more than 5 wt %, further not more than 2 wt %, andparticularly not more than 1 wt %. Therefore, no cost increase is causedby disposition and/or separation of coarse particles, and no reductionoccurs in physical properties of the water absorbing agent bypulverization of the coarse particles. Note here that the coarseparticles can be distinguished in the following manner. With use of aJIS standard sieve having a mesh opening size of 850 μm, the amounts of850 μm-on particles (i.e., the amounts of particles remaining on the850-μm sieve) of (i) a water absorbing resin powder to which water hasnot yet been added and of (ii) a water absorbing agent to which waterhas been added are measured. Then, the amount of 850 μm-on particles(i.e., the amount of particles remaining the 850-μm sieve) [wt %], whichamount is a difference between the amounts obtained before and afterwater is added, is calculated.

According to the present invention, an apparatus to be used for mixingthe metallic soap or the aqueous dispersion of the metallic soap withthe water absorbing resin is for example, but not limited to, acylindrical mixer, a screw-type mixer, a screw-type extruder,Turbulizer, a Nauter mixer, a V-shaped mixer, a ribbon mixer, adouble-arm kneader, a fluidization mixer, an air mixer, a rotating discmixer, a roll mixer, a tumbling mixer, a Loedige mixer, or the like. Themethod of mixing is for example a batch method, a continuous method, ora combination of the batch and continuous methods. In view of industrialproductivity, the continuous method is more preferable.

According to the particulate water absorbing agent production method 1(and further the production method 2) of the present invention, themetallic soap or the aqueous dispersion of the metallic soap is mixedwith the water absorbing resin under the following conditions.

That is, although a generally used mixing apparatus is a rotating mixer,the rotation speed of the mixing apparatus is not particularly limitedprovided that the water absorbing resin is not damaged. Specifically,the rotation speed is preferably between 5 rpm and 3000 rpm, morepreferably between 10 rpm and 500 rpm, and further preferably between 15rpm and 300 rpm. A rotation speed of more than 3000 rpm is notpreferable, because powder dust of the water absorbing resin isgenerated and water absorbing properties are reduced. On the other hand,a rotation speed of less than 5 rpm is not preferable, because materialsare not thoroughly mixed and a desired effect of improving fluidityafter moisture absorption cannot be obtained.

The temperature of a water absorbing resin powder before being mixedwith the metallic soap or with the aqueous dispersion of the metallicsoap is, but not particularly limited to, between room temperature and120° C., more preferably between 50° C. and 100° C., and furtherpreferably between 50° C. and 80° C. A temperature of the waterabsorbing resin powder of more than 120° C. is not preferable for thefollowing reason. That is, the added water evaporates, therebynecessitating addition of a large amount of water to achieve a desiredmoisture content and further increasing load applied on the mixingapparatus.

The mixing time during which the metallic soap or the aqueous dispersionof the metallic soap is mixed with the water absorbing resin is, but notparticularly limited to, preferably between 1 second and 20 minutes,more preferably between 10 seconds and 10 minutes, and furtherpreferably between 20 seconds and 5 minutes. A mixing time of more than20 minutes is not preferable for the following reason. That is, theresulting effect is not so good relatively to the period of time and, onthe contrary, the water absorbing resin turns into a powder state. Thiscauses an increase in the amount of fine powder (fine particles that canpass through a sieve having a mesh opening size of 150 μm) and anincrease in the amount of powder dust.

As is clear from above, the most preferable mixing condition to obtain aparticulate water absorbing agent of the present invention is asfollows: the temperature of a water absorbing resin powder is between50° C. and 80° C., the rotation speed of a mixing apparatus is between30 rpm and 300 rpm, and the mixing time is between 20 seconds and 5minutes. A particulate water absorbing agent obtained under thiscondition is excellent in handleability, and does not cause a problem ofadhesion or aggregation etc. Therefore, it is not necessary to carry outthe drying step for improving handleability.

(b) Metallic Soap

According to the second and third (and further the first and fourth)water absorbing agents and the water absorbing agent production methods1 and 2 (and further the production method 3), a metallic soap usable inthe present invention means an organic acid polyvalent metal salt, whichis a metal salt other than alkali metal salts such as a fatty acid, apetroleum acid and a polyacid each having 7 or more carbon atoms. Themetallic soap to be used generally contains a polyvalent metalcation(s), and is not or hardly soluble in water. According to the waterabsorbing agent production methods 1 and 2 using a metallic soap, aresulting water absorbing agent will contain a predetermined amount of apolyvalent metal cation(s) originating from a metallic soap or apolyvalent metal compound. Note here that the metallic soap and awater-soluble polyvalent metal compound (described later) may remain inthe water absorbing agent or change by a reaction with the waterabsorbing resin. Note, however, that the polyvalent metal cation(s)originating from the metallic soap or the water-soluble polyvalent metalcompound will be contained in the resulting water absorbing agent,usually almost all of them will be contained in the resulting waterabsorbing agent. For example, the amount of the polyvalent metalcation(s) is between 0.001 wt % and 5 wt %, and further between 0.005 wt% and 3 wt %. These amounts can be easily determined by the methoddescribed later.

An organic acid constituting the metallic soap is not particularlylimited, provided that the organic acid is the one that forms a saltwith a polyvalent metal. The organic acid is preferably an organiccarboxylic acid, an organic sulfonic acid, or an organic sulfinic acid.The organic acid is particularly preferably an organic carboxylic acidhaving a carboxyl group within a molecule.

The number of carbon atoms in the organic acid is preferably not lessthan 7, more preferably between 7 and 20, and further preferably between12 and 20. An organic acid having less than 7 carbon atoms is notpreferable, because the solubility of the metallic soap in water isincreased and the metallic soap may seep into urine or blood etc. when adisposable diaper or an absorbent core etc. is in use. Further, forexample in a case where an acid such as an oxalic acid or a citric acidis used, a polyvalent metal salt constituted by such acids has a highdegree of hardness. This may cause a reduction in water absorbingproperties upon mechanical shock. Further, using an oxalic acid is notpreferable in view of safety. Further, using an organic acid having morethan 20 carbon atoms is not preferable, because such an organic acid isdifficult to obtain and is expensive.

The organic carboxylic acid is not particularly limited provided thatthe organic carboxylic acid is a saturated or unsaturated aliphaticcarboxylic acid or a saturated or unsaturated aromatic carboxylic acid,and may have a substituent other than carboxyl groups, for example ahydroxyl group or a halogen atom etc. Further, the organic carboxylicacid may be a polyhydric carboxylic acid having a plurality of carboxylgroups within a molecule, and is more preferably a monocarboxylic acid.Specific examples of the organic carboxylic acid include: linear fattyacids and branched fatty acids such as octylic acid, octynoic acid,decanoic acid, lauric acid, myristic acid, palmitic acid, oleic acid,stearic acid, beef fatty acid, and hardened castor oil fatty acid;petroleum acids such as benzoic acid, naphthenic acid, naphthoic acid,and naphthoxyacetic acid; and polyacids such as poly(meth)acrylic acidand polysulfonic acid.

Out of those described above, a long-chain fatty acid such as octylicacid, octynoic acid, decanoic acid, lauric acid, myristic acid, palmiticacid, oleic acid, stearic acid, beef fatty acid, or hardened castor oilfatty acid is preferable; a long-chain saturated fatty acid having nounsaturated bond within a molecule such as octylic acid, decanoic acid,lauric acid, myristic acid, palmitic acid or stearic acid is morepreferable; and a C₁₂-C₂₀ long-chain saturated fatty acid having nounsaturated bond within a molecule such as lauric acid, myristic acid,palmitic acid or stearic acid is most preferable.

It should be noted that using a fatty acid having an unsaturated bond asthe organic carboxylic acid is not preferable, because a particulatewater absorbing agent of the present invention deteriorates in colortone or generates an odor etc. when subjected to heat or oxidationduring storage.

A metal salt constituting the metallic soap is not particularly limited,provided that the metal salt is other than alkali metal salts, i.e., apolyvalent metal salt such as an alkali earth metal salt or a transitionmetal salt etc. Note however that, in view of availability, barium salt,calcium salt, magnesium salt, aluminium salt, and zinc salt arepreferable; and calcium salt, magnesium salt, zinc salt, and aluminiumsalt are more preferable.

Accordingly, specific examples of the metallic soap include: calciumlaurate, magnesium laurate, zinc laurate, aluminium laurate, calciummyristate, magnesium myristate, zinc myristate, aluminium myristate,calcium palmitate, magnesium palmitate, zinc palmitate, aluminumpalmitate, calcium stearate, magnesium stearate, zinc stearate andaluminum stearate. Note, however, that the metallic soap is not limitedto the above, and any organic acid can be used in combination with ametal salt. These metallic soaps may be used solely or two or moremetallic soaps may be used in combination.

Further, part of the metallic soap may be a hydroxide or the like.Specifically, the metallic soap may have a salt structure represented byfor example (Organic Acid) _(x)M^(n+)(OH)_(n-x), wherein M^(n+)represents a n− valent metal ion, x is an integer between 1 and n, and nis an integer of 2 or greater.

According to the metallic soap in accordance with the present invention,not all of the acid radicals necessarily have to be salts. The metallicsoap may contain a few organic acid(s) or may contain excess polyvalentmetal. Note, however, that it is preferable that the metallic soap be asalt in which not less than mol % of acid radicals (carboxyl groups) areneutralized, more preferably 95 mol % to 105 mol %, further preferably98 mol % to 102 mol %, and particularly preferably 99 mol % to 101 mol %of acid radicals (carboxyl groups) are neutralized.

In a case where a polyacid such as polyacrylic acid is used as theorganic acid, it is preferable that not less than 95 mol %, morepreferably not less than 98 mol %, and further preferably not less than99 mol % of acid radicals (carboxyl groups) of the polyacid areneutralized to form a salt with a polyvalent metal. Further, theweight-average molecular weight of the polyacid is preferably between10,000 and 5,000,000, and more preferably between 50,000 and 1,000,000.

The metallic soap is in a powdery state, and its particle size is notparticularly limited. Usually, it is preferable that the particle sizebe smaller than the weight median particle size (D50) of the waterabsorbing resin particles. Specifically, the particle size of themetallic soap contained in the particulate water absorbing agent of thepresent invention is preferably more than 0 μm but less than 100 μm,more preferably not less than 0.01 μm but less than 50 μm, and furtherpreferably not less than 0.01 μm but less than 10 μm in median diameter.It should be noted that the median diameter means a particle diameter(cumulative average diameter) corresponding to 50% on a cumulativedistribution curve. The particle diameter is found by (i) plotting acumulative curve assuming that all the particles serve as a single groupand regarding the total volume of the group as 100% and (ii) finding apoint where the cumulative curve is 50%. The median diameter can bemeasured by dispersing a metallic soap in a solvent such as methanol andmeasuring the median diameter with use of a particle size distributionmeasuring apparatus LS-920 (HORIBA, Ltd.) or the like.

According to the present invention, the melting point of the metallicsoap is preferably between 20° C. and 250° C., more preferably between40° C. and 250° C., further preferably between 50° C. and 250° C.,further more preferably between 60° C. and 250° C., particularlypreferably between 70° C. and 250° C., and most preferably between 80°C. and 250° C. If the melting point of the metallic soap is higher than250° C., the metallic soap less adheres to the surface of the waterabsorbing resin, and a larger amount of metallic soap may fall off fromthe water absorbing resin. Further, a melting point of the metallic soapof lower than 20° C. is not preferable, because fluidity of a resultingparticulate water absorbing agent decreases and thus handleabilitydegreases. For this reason, when the particulate water absorbing agentis handled industrially, a method is employed by which to add heat andkeep the heat in a hopper for storing the particulate water absorbingagent or water absorbing resin, in a transport pipe and/or in a meteringfeeder etc. for the purpose of preventing the particulate waterabsorbing agent from absorbing moisture. In this case, usually thetemperature is kept (heated or the heat is kept) at between 30° C. and80° C.

Conventionally, most of additives such as polyethylene glycol andsurfactants generally have a low melting point or a low glass transitiontemperature. The additives are used for improving a powder propertyafter moisture absorption or a property of a powder having a moisturecontent of between 0 wt % and 20 wt %, in particular for improvingfluidity. Accordingly, there has been a problem in which, even if thefluidity of the particulate water absorbing agent is excellent at roomtemperature, the fluidity of a powder of the particulate water absorbingagent decreases and thus handleability decreases because the forgoingadditive is melted by heat added to a production apparatus and/ortransport line etc. during production of the particulate water absorbingagent or a disposable diaper etc. In this regard, according to thepresent invention, since a metallic soap having the foregoing meltingpoint is used, no reduction occurs in industrial handleability of theparticulate water absorbing agent when heating is carried out.

It should be noted that the melting point of the metallic soap may beactually measured or the melting point shown in Encyclopedia Chimica(Kagaku Daijiten, Edited by The Editing Committee for EncyclopediaChimica, published by KYORITSU SHUPPAN CO., LTD) etc. may be used. Forexample, according to Encyclopedia Chimica, the melting point of zincstearate is between 128° C. and 130° C., the melting point of aluminumstearate is 103° C., the melting point of calcium stearate is 180° C.,and the melting point of magnesium stearate is 200° C. Accordingly, suchmetallic soaps are suitably usable, because each of them has a meltingpoint most suitable for use in a particulate water absorbing agent ofthe present invention. In addition, appropriately selecting a metallicsoap to be used makes it possible to control the melting point so thatthe melting point ranges widely. Note however that, when putting ametallic soap into practical use, it is preferable to select and use ametallic soap having a melting point equal to or higher than atemperature at which the particulate water absorbing agent of thepresent invention is used.

The metallic soap is preferably insoluble or hardly soluble in deionizedwater at 25° C. For example, the solubility of the metallic soap ispreferably between 0 [g/L] and 10 [g/L], more preferably between 0 [g/L]and 5 [g/L], and further preferably between 0 [g/L] and 2 [g/L],relative to 1000 mL of deionized water. A solubility of the metallicsoap of more than 10 [g/L] is not preferable, because, as describedearlier, the metallic soap may seep into urine or blood etc.

When implementing the present invention, it is preferable to add ametallic soap and water in the form of an aqueous dispersion of ametallic soap. As used herein, the aqueous dispersion means a water inwhich a metallic soap is evenly dispersed with use of a dispersionstabilizer (described later), which water has fluidity. It is preferablethat the aqueous dispersion have a viscosity of up to 10000 cps (25°C.). Note, however, that an aqueous dispersion having low viscosity isalso usable in the present invention. An aqueous dispersion having aviscosity substantially the same as that of water can be used. Forexample, a slurry, a suspension liquid, and an emulsion etc. areencompassed in the aqueous dispersion in accordance with the presentinvention. Further, in a case where the metallic soap is produced in theform of an aqueous dispersion, the aqueous dispersion can be useddirectly without being dried or can be used after being concentrated ordiluted to some extent.

(c) Dispersion Stabilizer

According to the second and third (and further the first and fourth)water absorbing agents and the water absorbing agent production methods1 and 2 (and further the production method 3), in a case where ametallic soap is used in the form of an aqueous dispersion, a dispersionstabilizer is preferably used for the purpose of dispersing the metallicsoap in water stably without aggregation and particularly for causingthe metallic soap to be stable in colloidal form in water. Thedispersion stabilizer may be added separately from the metallic soap forthe propose of improving physical properties of a resulting waterabsorbing agent.

The dispersion stabilizer is not particularly limited, provided that ithas conventionally been used for stably dispersing water-insoluble fineparticles in water. The dispersion stabilizer is for example awater-soluble polymer, a hydrophilic organic solvent, or a surfactantetc. (described later), and is preferably a surfactant. Further,according to the water absorbing agent production methods 1 and 2 (andfurther the production method 3), such a dispersion stabilizer used willremain on the surface of a resulting water absorbing agent or will becontained in the resulting water absorbing agent in a predeterminedamount. This further improves physical properties (e.g., shockresistance, transportability) of the resulting water absorbing agent.

(c-1) Water-Soluble Polymer

According to the second and third (and further the first and fourth)water absorbing agents and the water absorbing agent production methods1 and 2 (and further the production method 3) of the present invention,a water-soluble polymer used as a dispersion stabilizer for a metallicsoap is for example, but not particularly limited to, polyvinyl alcohol,starch, (carboxy)methyl cellulose, hydroxyethyl cellulose, polyacrylicacid (salt) or the like. The water-soluble polymer to be used ispreferably soluble in water (25° C.) in an amount of preferably not lessthan 1 wt %, more preferably not less than 5 wt %, and furtherpreferably not less than 10 wt %. The molecular weight of thewater-soluble polymer is preferably between 500 and 50,000,000, morepreferably between 1000 and 5,000,000, and further preferably between10,000 and 1,000,000.

(c-2) Hydrophilic Organic Solvent

According to the second and third (and further the first and fourth)water absorbing agents and the water absorbing agent production methods1 and 2 (and further the production method 3) of the present invention,a hydrophilic organic solvent used as a dispersion stabilizer for ametallic soap is for example, but not particularly limited to, ahydrophilic organic solvent etc. used in combination with thecrosslinking agent in the foregoing surface crosslinking. Specifically,polyhydric alcohols such as ethylene glycol, diethylene glycol,propylene glycol, triethylene glycol, tetraethylene glycol, polyethyleneglycol, 1,3-propanediol, dipropylene glycol,2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin,polyglycerin, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexane dimethanol,1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine,polyoxypropylene, oxyethylene-oxypropylene block copolymers,pentaerythritol, and sorbitol are preferable, C₂-C₁₀ polyhydric alcoholsare more preferable, and C₃-C₈ polyhydric alcohols are furtherpreferable. Further, hydroxy groups of such polyhydric alcohols may bepartially alkoxylated (e.g., methoxy polyethylene glycol).

Out of the above polyhydric alcohols, it is preferable to use ethyleneglycol, propylene glycol, propanediol, butanediol, pentanediol,hexanediol, glycerin and/or trimethylolpropane. These polyhydricalcohols may be used solely or two or more polyhydric alcohols may beused in combination.

(c-3) Surfactant

According to the second and third (and further the first and fourth)water absorbing agents and the water absorbing agent production methods1 and 2 (and further the production method 3) of the present invention,a surfactant used as a dispersion stabilizer for a metallic soap is notparticularly limited. Examples of the surfactant include:polyoxyalkylene alkyl ethers such as polyoxyethylene lauryl ether,polyoxyethylene cetyl ether, polyoxyethylene stearyl ether, andpolyoxyethylene oleyl ether; polyoxyalkylene alkylphenyl ethers such aspolyoxyethylene octylphenyl ether and polyoxyethylene nonylphenyl ether;polyoxyalkylene alkylamino ethers such as polyoxyethylene laurylaminoether and polyoxyethylene stearylamino ether; sorbitan fatty acid esterssuch as sorbitan monolaurate, sorbitan monopalmitate, sorbitanmonostearate, and sorbitan monooleate; polyoxyalkylene sorbitan fattyacid esters such as polyoxyethylene sorbitan monolaurate,polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitanmonostearate, and polyoxyethylene sorbitan monooleate; polyalkyleneglycol fatty acid esters such as polyethylene glycol monolaurate,polyethylene glycol monooleate, polyethylene glycol monostearate,polyethylene glycol dilaurate, and polyethylene glycol distearate;nonionic surfactants, for example glycerine fatty acid esters such asmonoglyceride laurate, monoglyceride stearate, and monoglyceride oleate;sulfuric ester salts such as sodium polyoxyethylene laurylether sulfate,sodium polyoxyethylene octylphenylether sulfate, sodium polyoxyethylenenonylphenylether sulfate, triethanolamine lauryl sulfate, sodium laurylsulfate, potassium lauryl sulfate, and ammonium lauryl sulfate;sulfonates such as sodium dodecylbenzenesulfonate, alkyl sodiumnaphthalene sulfonate, and dialkyl sulfo sodium succinate; anionicsurfactants, for example phosphoric acid ester salts such as alkylpotassium phosphate; and cationic surfactants, for example quarternaryammonium salts such as lauryl trimethylammonium chloride, stearyltrimethylammonium chloride, cetyl trimethylammonium chloride, andstearyl trimethylammonium chloride. These surfactants may be used solelyor two or more surfactants may be used in combination. Out of thesesurfactants, it is preferable to use an nonionic surfactant or ananionic surfactant.

It is preferable to use a surfactant as a dispersion stabilizer for ametallic soap. A method for producing an aqueous dispersion of ametallic soap in which a surfactant is used as a dispersion stabilizermay be for example a method described in Japanese Patent ApplicationPublication “Tokukaisho No. 59-219400” or a method described inTranslation of PCT Patent Application “Tokuhyohei, No. 4-7260”. Further,commercially available aqueous dispersions such as Zinc Stearate N(manufactured by NOF CORPORATION.), Hidorin Z-7-30, Hidorin O-128(manufactured by CHUKYO YUSHI CO., LTD), Afco-Disper ZD, Afco-Disper C(manufactured by ADEKA CHEMICAL SUPPLY CO., LTD.) and/or the like may beused directly.

(d) Controlling Moisture Content

The second and third (and further the first and fourth) particulatewater absorbing agents in accordance with the present invention can beobtained by adding a metallic soap and water to a water absorbing resinby the foregoing method so as to control the moisture content to apredetermined moisture content. The predetermined moisture content isbetween 5 wt % and 20 wt %, preferably between 5 wt % and 18 wt %, morepreferably between 6 wt % and 17 wt %, further preferably between 7 wt %and 15 wt %, and particularly preferably between 8 wt % and 13 wt %.

How to control the moisture content is not particularly limited. Forexample, the amount of water or the amount of an aqueous dispersion of ametallic soap to be added may be controlled as appropriate depending onthe moisture content of a water absorbing resin to be used.Alternatively, a predetermined amount of water may be allowed to remainin a particulate water absorbing agent.

Alternatively, after the foregoing amount of water or the foregoingamount of an aqueous dispersion of a metallic soap is added to a waterabsorbing resin, the water absorbing resin is dried by heating or driedunder reduced pressure so that its moisture content is controlled.

Alternatively, after water or an aqueous dispersion of a metallic soapis added to a water absorbing resin, the absorbing resin may be heatedand/or dried as needed. In a case where the moisture content is to becontrolled by drying by heating etc., infiltration of water into thewater absorbing resin is promoted, and the surface is dried so that thewater absorbing resin can quickly turn into particles.

Alternatively, in a case where a metallic soap is added together withwater to a hydrous gel or a case where an aqueous dispersion of ametallic soap is added to a hydrous gel, the heating condition etc. ofthe drying step or the surface crosslinking step are determined asappropriate so that the moisture content becomes a predeterminedmoisture content of the particulate water absorbing agent.

By controlling the moisture content of the particulate water absorbingagent so that the moisture content falls within the above range, it ispossible to obtain a particulate absorbing agent which has improvedfluidity after moisture absorption and improved water absorbing property(AAP) and is excellent in shock resistance (abrasion resistance).Further, it is possible to suppress the particulate water absorbingagent from being charged, and thus possible to dramatically improvehandleability of powder.

In a case where the moisture content is controlled by drying by heating,the heating temperature is preferably between 30° C. and 100° C., morepreferably between 50° C. and 90° C., and further preferably between 60°C. and 80° C. The heating time is preferably between 1 second and 3hours, and further is determined as appropriate to between 1 minute and1 hour. A water absorbing agent that has entered into a powder state bybeing heated as needed may be used directly, or may be furtherdisintegrated, classified and/or granulated as needed.

(e) Particle Size Control Step (Granulation Step, Particle Sizing Step,Fine Powder Collection Step)

Each of the second and third (and further the first and fourth)particulate water absorbing agents of the present invention may besubjected to an appropriate process(es) such as the particle sizingstep, granulation step and/or fine powder collection step etc. so thatthe particulate water absorbing agent has a predetermined particle size(described later), as needed. It should be noted that the particle sizecontrol step is for example the step described in US Patent ApplicationPublication, No. 2004/181031, US Patent Application Publication, No.2004/242761, or US Patent Application Publication, No. 2006/247351 etc.

(f) Other Component Contained in Particulate Water Absorbing Agent

To the second and third (and further the first and fourth) particulatewater absorbing agents of the present invention, for the purpose offurther imparting various properties, insoluble fine particles such asan inorganic powder or a hydrophilic solvent such as water may be addedin addition to the foregoing components (water absorbing resin, organicpolyvalent metal salt, internal crosslinking agent, polymerizationinitiator, surface crosslinking agent etc.), and the water absorbingresin etc. may be granulated.

Specific examples of the inorganic powder include: metallic oxides suchas silicon dioxide and titanium oxide; silicic acids (salts) such asnatural zeolite and synthetic zeolite; kaolin; talc; clay; andbentonite. Out of these, silicon dioxide and silicic acids (salts) aremore preferable, and silicon dioxide and silicic acids (salts) eachhaving a mean particle size of not more than 200 μm measured by Coultercounter are preferable.

The amount of the inorganic powder to be added depends on a combinationof the inorganic powder with various components contained in theparticulate water absorbing agent. The amount of the inorganic powder isnot particularly limited provided that the amount is between 0 an 6parts by weight relative to 100 parts by weight of the water absorbingresin. The amount of the inorganic powder is more preferably between0.001 and 5 parts by weight, and particularly preferably between 0.01and 3 parts by weight. If the amount of the inorganic powder to be addedis outside the above range, the shock absorption capacity of themetallic soap may become insufficient, and it may become difficult toprevent water absorbing properties from decreasing upon shock.

How to mix the water absorbing resin and the inorganic powder is notparticularly limited. The inorganic powder may be mixed with the waterabsorbing resin simultaneously with or separately from an aqueousdispersion of a metallic soap. For example, it is possible to employ adry blending method by which to blend powders or a wet blending methodetc. The dry blending method is more preferable.

Further, the present invention may further include as needed a step ofadding, simultaneously with or separately from an aqueous dispersion ofa metallic soap, various additives as needed to impart various functionsto a particulate water absorbing agent of the present invention.Examples of the various additives include: deodorizers, antimicrobialagents, perfumes, foaming agents, pigments, dyes, plasticizers,adhesives, surfactants, fertilizers, oxidizing agents, reducing agents,water, salts, chelating agents, germicides, hydrophilic polymers such aspolyethylene glycol and polyethylene imine, hydrophobic polymers such asparaffin, thermoplastic resins such as polyethylene and polypropylene,and thermosetting resins such as polyester resins and urea resins. Theamount of an additive to be added may be between 0 and 30 parts byweight, is preferably between 0 and 10 parts by weight, and morepreferably between 0 and 1 part by weight, relative to 100 parts byweight of the water absorbing resin particles.

[6] Physical Properties of the First to Third (and Further the Fourth)Particulate Water Absorbing Agents According to the First to ThirdEmbodiments

<Moisture Content and Polyvalent Metal Cation>

The third (and further the first, second and fourth) particulate waterabsorbing agent(s) of the present invention is a particulate waterabsorbing agent which contains a water absorbing resin and a metallicsoap and which has a moisture content of between 5 wt % and 20 wt %,preferably between 5 wt % and 18 wt %, and more preferably between 6 wt% and 18 wt %. The particulate water absorbing agent shows excellentfluidity after moisture absorption and excellent AAP, and in addition,shows excellent stability to shock.

<Particle Size>

Further, during production of the particulate water absorbing agent ofthe present invention, an extremely small number of particles are boundto one another (e.g., the percentage of particles having a particle sizeof not less than 850 μm in a hardened product is extremely small) whenwater is added, and the particle size is easily controlled to a desiredparticle size. The metallic soap and the dispersion stabilizer(surfactant) are contained inside the water absorbing resin or in thesurface of the water absorbing resin, preferably contained at least inthe surfaces of particles.

Specifically, the particulate water absorbing agent of the presentinvention (i) includes particles of not less than 106 μm but less than850 μm in diameter in an amount between 90 wt % and 100 wt % relative tothe total amount of particles, and particles of not less than 300 μm indiameter in an amount of not less than 60 wt % relative to the totalamount of particles, (ii) has an absorbency against pressure (AAP 4.83kPa) of not less than 10 [g/g], and (iii) has a fluidity after moistureabsorption of between 0 wt % and 10 wt %. More preferably, theparticulate water absorbing agent of the present invention shows theforegoing absorbency against pressure (4.83 kPa) and the foregoingfluidity after moisture absorption, and has a dusting rate of preferablybetween 0 wt % and 1.0 wt %, more preferably a dusting rate fallingwithin the range described later.

The absorbency against pressure and stability to shock are physicalproperties that are in a trade-off relationship, and thus an increase inone of them decreases the other. That is, if the stability to shock isincreased, the absorption against pressure decreases, and thus thefluidity after moisture absorption dramatically decreases. On the otherhand, if the absorbency against pressure is increased, the stability toshock decreases. According to a conventional technique, it is notpossible to improve both of these physical properties that are in atrade-off relationship. In this regard, according to the presentinvention, it is possible to improve both of these physical propertiesthat are in a trade-off relationship.

The following description discusses preferable physical properties ofthe first to third (and further the fourth) particulate water absorbingagents of the present invention.

(6-1) Absorbency Against Pressure (AAP 4.83 kPa)

According to the foregoing production methods, no significant change iscaused in physical properties by addition of a metallic soap. Therefore,it is possible to obtain a particulate water absorbing agent having highabsorbency against pressure. The second and third (and further the firstand fourth) particulate water absorbing agents of the present inventioneach have an absorbency against pressure falling within the rangedescribed earlier (in the (4-3)).

If absorbency (AAP) against a pressure of 4.83 kPa is less than 10[g/g], it may be impossible to obtain a water absorbing agent in whichthe amount of liquid squeezed out (so-called Re-Wet) is small when thewater absorbing agent is used in an absorbent core and pressure isapplied to the absorbent core.

The reason why the absorbency against pressure under a load of 4.83 kPais used is that it is assumed that, in a case where the particulatewater absorbing agent of the present invention is used as a sanitarymaterial such as a disposable diaper, such a load is applied to theparticulate water absorbing agent when an infant is lying down or issitting upright.

(6-2) Fluidity after Moisture Absorption

The “fluidity after moisture absorption” of the present invention isdetermined by evaluating blocking, caking, or fluidity of a powder underthe condition where a particulate water absorbing agent is allowed tostand at a temperature of 25° C. with a relative humidity of 90%. The“fluidity after moisture absorption” is determined based on “blockingrate after moisture absorption”.

Specifically how the blocking rate after moisture absorption is measured(evaluated) is described later. The blocking rate after moistureabsorption of the first to fourth particulate water absorbing agents ofthe present invention is preferably between 0 wt % and 10 wt %, morepreferably between 0 wt % and 5 wt %, and further preferably between 0wt % and 2 wt %. If the blocking rate after moisture absorption is morethan 10 wt %, the particulate water absorbing agent is difficult tohandle in humid conditions. Accordingly, there may be a problem inwhich, during production of a thin absorbent core for sanitary materialsetc., the water absorbing agent may aggregate in a transport pipe in aproduction plant and thus the transport pipe clogs and/or the waterabsorbing agent cannot be evenly mixed with hydrophilic tissue.

The foregoing blocking rate after moisture absorption is achieved bycontrolling the particle size of the water absorbing resin and thenusing a metallic soap or a metal compound (described later).

(6-3) CRC and Extractable Content (Ext)

The first to fourth particulate water absorbing agents of the presentinvention each preferably have a centrifugal retention capacity (CRC)falling within the range described earlier (in the (4-2)).

If the centrifugal retention capacity (CRC) is less than 10 [g/g], thewater absorbency is so low that a resulting water absorbing agent is notsuitable for use in sanitary materials such as a disposable diaper. Onthe other hand, if the centrifugal retention capacity (CRC) is more than60 [g/g], it may be impossible to obtain a particulate water absorbingagent which is excellent in rate of liquid absorption into an absorbentcore when the particulate water absorbing agent is used in the absorbentcore.

The present invention provides a suitable absorbing article by achievingthe forgoing CRC. According to foregoing production methods, nosignificant change is caused in CRC by addition of a metallic soap.Therefore, it is possible to obtain a particulate water absorbing agenthaving the foregoing particle size. Note, however, that classificationand/or granulation may be carried out in the production methods asneeded.

Further, the amount of extractables (Extr), which is specified in ERT470.1-02, of the water absorbing resin is not more than 50 wt %,preferably not more than 30 wt %, and more preferably not more than 20wt %.

(6-4) Dusting Rate

The dusting rate of each of the first to fourth particulate waterabsorbing agents of the present invention is obtained by evaluating theamount of fine powder generated in the production and transport of theparticulate water absorbing agent.

The dusting rate of a particulate water absorbing agent of the presentinvention is between 0 wt % and 1.0 wt %, preferably between 0 wt % and0.8 wt %, and more preferably between 0 wt % and 0.5 wt %. If thedusting rate is more than 1.0 wt %, there may be a problem in which adeterioration in operating environments is caused by powder dustgenerated during the production and transport of the particulate waterabsorbing agent.

(6-5) Particle Size

Each of the first to fourth particulate water absorbing agents of thepresent invention includes (i) particles of not less than 106 μm butless than 850 μm in diameter in an amount between 90 wt % and 100 wt %relative to the total amount of particles and (ii) particles of not lessthan 300 μm in diameter in an amount of not less than 60 wt % relativeto the total amount of particles.

In particular, the particulate water absorbing agent preferably includesparticles of not less than 106 μm but less than 850 μm in diameter in anamount between 95 wt % and 100 wt %, particularly preferably between 98wt % and 100 wt %, relative to the total amount of particles. Further,the particulate water absorbing agent preferably includes particles ofnot less than 300 μm in diameter in an amount of preferably between 65wt % and 100 wt %, more preferably between wt % and 100 wt %, andparticularly preferably between 75 wt % and 100 wt %, relative to thetotal amount of particles.

Furthermore, the weight median particle size (D50) of the particulatewater absorbing agent is preferably between 200 μm and 700 μm, furtherpreferably between 300 μm and 600 μm, and particularly preferablybetween 400 μm and 500 μm. Moreover, the logarithmic standard deviation(σζ), which is an index of uniformity of particle size distribution, ofthe particulate water absorbing agent is preferably between 0 and 0.40,more preferably between 0 and 0.35, and most preferably between 0 and0.30.

If a particulate water absorbing agent contains particles of not lessthan 850 μm in diameter in an amount of more than 10 wt % relative tothe total amount of particles, such a particulate water absorbing agentwill cause discomfort in a user, e.g., cause a user a feeling of aforeign body or a feeling of roughness, when used for producing asanitary material such as a disposable diaper. Further, if a particulatewater absorbing agent contains particles of less than 106 μm in diameterin an amount of more than 10 wt % relative to the total amount ofparticles and/or has a logarithmic standard deviation (σζ) of more than0.40, such a particulate water absorbing agent is not preferable becausemany problems occur such as dramatic reductions in absorbency againstpressure and fluidity after moisture absorption, deterioration inoperating environments due to powder dust generated during production ofsanitary materials such as disposable diapers, and an increase insegregation due to wide particle size distribution.

(6-6) Moisture Content of Particulate Water Absorbing Agent

The moisture content of each of the first, third and fourth (and furtherthe second) particulate water absorbing agents in accordance with thefirst to third embodiments of the present invention is between 5 wt %and 20 wt %, preferably between 5 wt % and 18 wt %, more preferablybetween 6 wt % and 18 wt %, further preferably between 7 wt % and 15 wt%, and particularly preferably between 8 wt % and 13 wt %. It themoisture content is less than 5 wt %, such a particulate water absorbingagent is not preferable because sufficient stability to shock is notachieved and, in addition, the fluidity after moisture absorptiondecreases and thus handleability decreases. Further, if the moisturecontent is more than 20 wt %, such a particulate water absorbing agentis not preferable because water absorbing properties (e.g., AAP, CRC)decrease and the fluidity after moisture absorption decreases.

[7] Fourth Embodiment

The fourth embodiment of the present invention is a method (waterabsorbing agent production method 3) for producing a particulate waterabsorbing agent containing a water absorbing resin as a main component,which method includes: surface-treating the water absorbing resin by asurface treatment method including the steps of (a) mixing an acidradical-containing radical-polymerizable monomer, a polyvalent metalcompound and water with the water absorbing resin and

(b) polymerizing the acid radical-containing radical-polymerizablemonomer.

It is preferable that the method be:

(1) a production method in which the step (b) includes irradiating amixture obtained in the step (a) with activating light and/or heatingthe mixture obtained;

(2) a production method in which a degree of neutralization of the acidradical-containing radical-polymerizable monomer is between 0 mol % and60 mol %;

(3) a production method in which the acid radical-containingradical-polymerizable monomer is a (meth)acrylic acid (salt);

(4) a production method in which the acid radical-containingradical-polymerizable monomer further contains a polyfunctional organiccrosslinking agent;

(5) a production method in which, in the step (a), at least one type ofradical polymerization initiator selected from the group consisting ofpersulfates, hydrogen peroxides and water-soluble azo compounds isfurther mixed with the water absorbing resin;

(6) a production method in which, in the step (a), a photodegradablepolymerization initiator is further mixed with the water absorbingresin;

(7) a production method in which the water absorbing resin is a powderypolyacrylic acid (salt) water absorbing resin obtained by polymerizing amonomer containing a (meth)acrylic acid (salt) as a main component;

(8) a production method in which the polyvalent metal compound is awater-soluble polyvalent typical metal salt;

(9) a production method in which the polyvalent metal compound is aninorganic or organic water-soluble aluminum salt;

(10) a production method in which the acid radical-containingradical-polymerizable monomer is polymerized in the presence of water inan amount between 5 wt % and 20 wt % relative to the amount of the waterabsorbing resin;

(11) a production method in which the acid radical-containingradical-polymerizable monomer is mixed in an amount between 0.1 and 20parts by weight relative to 100 parts by weight of the water absorbingresin, the polyvalent metal compound is mixed in an amount between 0.01and 10 parts by weight relative to 100 parts by weight of the waterabsorbing resin, and the water is mixed in an amount between 1 and 50parts by weight, relative to 100 parts by weight of the water absorbingresin;(12) a method for producing a particulate water absorbing agentcontaining, as a main component, a surface-treated polyacrylic acid(salt) water absorbing resin, wherein the absorbency without pressure(CRC) is not less than 28 (g/g), the absorbency against pressure (AAP4.83 kPa) is not less than 10 (g/g), the vertical diffusion absorbencyunder pressure (VDAUP) is not less than 20 g, and the moisture contentis between 5 wt % and 20 wt %;(13) a method for producing a water absorbing agent by furthersurface-treating the water absorbing agent obtained by the method (12)with a polyvalent metal;(14) a method for producing a water absorbing agent by furthersurface-treating the water absorbing agent obtained by the method (12)or (13) with a polyvalent metal that is other than a metallic soap;(15) a method for producing a water absorbing agent by furthersurface-treating any one of the water absorbing agents obtained by anyone of the methods (12) through (14) with a water-soluble aluminum salt;(16) a method for producing any one of the water absorbing agentsobtained by any of the methods (12) through (15), in which waterabsorbing agent a residual monomer content is not more than 500 ppm;and/or(17) a method for producing any one of the water absorbing agentsobtained by any of the methods (12) through (16), which water absorbingagent is a powder in a form of irregular fragments.

The following description describes in detail a method ofsurface-treating a water absorbing resin in accordance with the presentinvention. Note, however, that the scope of the present invention is notrestricted by the following descriptions. Those other than the followingdescriptions, i.e., appropriate modifications of the followingdescriptions, can be implemented provided that the objects of thepresent inventions are attained.

[7-A] Materials for Fourth Embodiment (Water Absorbing Agent ProductionMethod 3)

According to the water absorbing agent production method 3, it ispossible to obtain the fourth water absorbing agent, and furtherpossible to obtain the first to third water absorbing agents. Thefollowing description discusses the water absorbing agent productionmethod 3 and the third water absorbing agent.

(1) Water Absorbing Resin

A water absorbing resin in accordance with the fourth embodiment of thepresent invention encompasses, as defined earlier, (i) a water absorbingresin powder (water absorbing resin precursor, base polymer) which is awater absorbing resin that has not yet been surface-treated norsurface-crosslinked and (ii) surface-crosslinked water absorbing resinparticles.

It should be noted that the water absorbing resin powder (also referredto as a base polymer) is produced by the method described in theforegoing (3-1) to (3-3) of [3]. Further, the water absorbing resinparticles are produced by the method described in the foregoing (3-4) of[3], and preferably has the properties described in the [4].

(2) Radical Polymerizable Compound (Surface-Treatment Agent for WaterAbsorbing Resin)

(2-1) Acid Radical-Containing Unsaturated Monomer

According to the production method 3 of the present invention, use of aradical polymerizable monomer containing an acid radical (acidradical-containing unsaturated monomer) is essential. Out of radicalpolymerizable monomers, a monomer containing an acid radical isexcellent in water absorbing properties. Examples of the acid radicalencompass carboxyl, sulfo, and phosphate groups.

The acid radical-containing radical-polymerizable monomer to be mixedwith the water absorbing resin in the present invention is preferably amonomer containing an acid radical, out of the foregoing unsaturatedethylene monomers. Specific examples of the acid radical-containingradical-polymerizable monomer include: (meth)acrylic acid,2-(meth)acryloyl ethanesulfonic acid, 2-(meth)acryloyl propanesulfonicacid, 2-(meth)acrylamide-2-methylpropanesulfonic acid, vinylsulfonicacid, styrenesulfonic acid and/or their salts. Out of these, in view ofwater absorbing properties, (meth)acrylic acid and2-(meth)acrylamide-2-methylpropanesulfonic acid are more preferable, andacrylic acid is particularly preferable. The amount of acrylic acid(salt) is between 50 mol % and 100 mol %, further preferably between 70mol % and 100 mol %, and particularly preferably between 90 mol % and100 mol %, relative to the total amount of monomers. The acidradical-containing radical-polymerizable monomer may be used solely or amixture of two or more types of acid radical-containingradical-polymerizable monomers may be used.

According to the production method 3 of the present invention, thedegree of neutralization of the acid radical-containingradical-polymerizable monomer is preferably low. The ratio of the degreeof neutralization of the monomer to that of the water absorbing resinfalls within the following range (the degree of neutralization of themonomer is preferably not more than 0.8 times, not more than 0.6 times,not more than 0.4 times, and not more than 0.2 times the degree ofneutralization of the water absorbing resin that has not yet beensurface-treated).

Further, the degree of neutralization of the monomer is preferablybetween 0 mol % and 80 mol %, more preferably between 0 mol % and 60 mol%, further preferably between 0 mol % and 40 mol %, further preferablybetween 0 mol % and 25 mol %, particularly preferably between 0 mol %and 15 mol %, and most preferably between 0 mol % and 10 mol %.

The smaller the degree of neutralization of the acid radical-containingradical-polymerizable monomer, the faster the reaction in the subsequentsurface treatment caused by irradiation with activating light and/or byheating. This makes it possible to obtain, at low temperatures within ashort period of time, a water absorbing resin having excellent waterabsorbing properties. For example, this provides a significant economiceffect in a case of large-scale production (preferably continuousproduction), e.g., in a case where the water absorbing resin is producedon a commercial scale of 1000 [kg/hr].

The degree of neutralization of the acid radical-containingradical-polymerizable monomer to be mixed is preferably not more than 80mol % relative to the degree of neutralization of the water absorbingresin that has not yet been surface-treated (in other words, not morethan 8/10 of the degree of neutralization of the water absorbing resin),more preferably not more than 60 mol % (not more than 6/10), furtherpreferably not more than 40 mol % (not more than 4/10), particularlypreferably not more than 20 mol % (not more than 2/10), and mostpreferably not more than 10 mol % (not more than 1/10).

It should be noted that the degree of neutralization means a ratio ofthe number of a neutralized acid radical(s) to the total number of acidradicals in the acid radical-containing radical-polymerizable monomer.The “total number of acid radicals in the acid radical-containingradical-polymerizable monomer” and the “number of neutralized acidradical” mean, in a case where two or more types of acidradical-containing radical-polymerizable monomers are used, the totalnumber of acid radicals and the number of a neutralized acid radical(s)in each acid radical-containing polymerizable compound, respectively.For example, in a case of using an acid radical-containing radicalpolymerizable compound containing acrylic acid and sodium acrylate at amolar ratio of 1:1, the degree of neutralization of the acidradical-containing radical polymerizable compound is 50 mol %. That is,according to this method, the present invention provides a waterabsorbing resin in which preferably the degree of neutralization islower in the surface than inside of the polyacrylic acid (salt) waterabsorbing resin powder.

In a case where the acid radical-containing radical-polymerizablemonomer is neutralized (in a case where it is in the form of a salt),the compound is preferably a nomovalent salt selected from alkali metalsalts, ammonium salts, and amine salts. The compound is more preferablyan alkali metal salt, and particularly preferably a salt selected fromsodium salts, lithium salts, and potassium salts.

The amount of the acid radical-containing radical-polymerizable monomerto be used is, but not particularly limited to, preferably between 0.1and 20 parts by weight, more preferably between 0.5 and 15 parts byweight, further preferably between 1 and 10 parts by weight,particularly preferably between 1.5 and 8 parts by weight, and mostpreferably between 2 and 7 parts by weight, relative to 100 parts byweight of the water absorbing resin. Since the amount of the acidradical-containing radical-polymerizable monomer falls within theforegoing range, the effect of the present invention becomes noticeable.Since the amount of the acid radical-containing radical-polymerizablemonomer is not less than 0.1 parts by weight, the water absorbingproperty under pressure of the water absorbing resin is sufficientlyimproved. Further, since the amount of the acid radical-containingradical-polymerizable monomer is not more than 20 parts by weight, nosignificant reduction occurs in water absorbency of a resultingsurface-treated water absorbing resin.

(2-2) Monomer

According to the production method 3 of the present invention, anunsaturated ethylene monomer that can be used in addition to the acidradical-containing radical-polymerizable monomer is an unsaturatedmonomer usable in polymerization of the water absorbing resin (basepolymer). Specific examples of the unsaturated ethylene monomer include:nonionic hydrophilic group-containing monomers such as (meth)acrylamide,N-substituted (meth)acrylamide, 2-hydroxyethyl (meth)acrylate, and2-hydroxypropyl (meth)acrylate; and amino group-containing unsaturatedmonomers such as N,N-dimethylaminoethyl (meth)acrylate,N,N-diethylaminoethyl (meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and N,N-dimethylaminopropyl (meth)acrylamide; and theirquaternary compounds.

The amount of the unsaturated ethylene monomer in such a case can beselected as appropriate depending on a desired property. Note, however,that the amount of the unsaturated ethylene monomer is preferablybetween 0 wt % and 100 wt %, more preferably between 1 wt % and 50 wt %,relative to 100 wt % of the acid radical-containing radial-polymerizablemonomer.

Further, by appropriately selecting a radical polymerizable compound tobe mixed with the water absorbing resin, it is possible to impart, tothe surfaces of surface-treated water absorbing resin particles, variousproperties such as a hydrophilic property, hydrophobic property,adhesion property, biocompatibility, and/or the like. Patent Literature30 describes an unsaturated ethylene monomer that imparts a hydrophilicproperty to the surfaces of water absorbing resin particles.

(2-3) Crosslinking Agent

According to the production method 3 of the present invention, it ispreferable that a crosslinking agent (for example, a crosslinking agentthat reacts with a carboxyl group) that is copolymerized with or reactswith a monomer (particularly acrylic acid) be used. That is, acrosslinkable unsaturated monomer that is crosslinkable with an acrylicacid and/or a crosslinking agent reactive with an acrylic acid(non-polymerizable crosslinking agent) are/is used.

As a crosslinkable unsaturated monomer that is copolymerizable with anacrylic acid, it is possible to use a crosslinking agent having aplurality of groups, in particular 2 to 10 groups, within a molecule.Examples of the groups include acrylate, acrylamide, and allyl groups.Further, as a crosslinking agent that is reactive with acrylic acid, itis possible to use a crosslinking agent having a plurality of groups, inparticular 2 to 10 groups, within a molecule. Examples of the groupsinclude epoxy, hydroxy, and amino groups. Each of the above can be usedas a surface crosslinking agent. Further, a crosslinking agent havingboth of the above properties is for example a crosslinking agent havingboth a polymerizable group and a reactive group within a molecule, e.g.,hydroxyethyl acrylate.

The crosslinkable unsaturated monomer is for example, but notparticularly limited to, a crosslinking agent usable in polymerizationof the water absorbing resin. Specific example of the crosslinkableunsaturated monomer includes a monomer to be used as an internalcrosslinking agent in production of the water absorbing resin. Out ofthese, it is preferable to use polyethylene glycol diacrylate,trimethylolpropane tri(meth)acrylate, N,N′-methylenebis(meth)acrylamide, glycerin acrylate methacrylate and/or the like, inwhich the average number of ethylene oxides is between 2 and 50. Inorder for the crosslinkable unsaturated monomer to be efficientlycopolymerized with an unsaturated ethylene monomer on the surface of thewater absorbing resin, it is desirable that the crosslinkableunsaturated monomer and the unsaturated ethylene monomer have similardispersibility in the water absorbing resin during the step of mixingwith the water absorbing resin. To this end, it is desirable that thecrosslinkable unsaturated monomer (polymerizable crosslinking agent) besimilar in molecular weight and hydrophilic property to the unsaturatedethylene monomer to be used. Alternatively, it is possible to use forexample a polyhydric alcohol or a polyhydric glycidyl compoundexemplified as a surface crosslinking agent. That is, it is possible touse for example a non-polymerizable crosslinking agent such as ethyleneglycol diglycidyl ether, (poly)glycerol glycidyl ether, or polyethyleneglycol diglycidyl ether.

Two or more of the internal crosslinking agents may be used incombination. The amount of an internal crosslinking agent (preferably acrosslinkable unsaturated monomer) to be used can be selected asappropriate depending on a desired property. The amount of the internalcrosslinking agent is preferably between 0 wt % and 20 wt %, morepreferably between 0.1 wt % and 10 wt %, and most preferably between 0.5wt % and 5 wt %, relative to 100 wt % of a radical polymerizablecompound. Using also the internal crosslinking agent (particularlypreferably a crosslinkable unsaturated monomer) makes it possible tofurther improve the vertical diffusion absorbency under pressure (VDAUP)and the absorbency against pressure (AAP). The reason why the VDAUP andthe AAP are improved is unknown, but the reason is probably that thewater-soluble unsaturated ethylene monomer forms a crosslinked structureduring polymerization and the crosslinked structure is introduced intothe surface of the water absorbing resin.

It should be noted here that the molar composition ratio of the internalcrosslinking agent may be the same as or different from that of thewater absorbing resin serving as a base polymer. Note, however, that itis preferable that the amount of crosslinkable monomers be larger thanthe amount of unsaturated ethylene monomers in a base polymer. Forexample, it is preferable that the amount of crosslinkable monomers be1.01 to 1000 times the amount of the unsaturated ethylene monomers inthe base polymer in mol %.

The amount of the crosslinkable unsaturated monomer to be used ispreferably between 0.001 mol % and 100 mol %, more preferably between0.01 mol % and 50 mol %, further preferably between 0.05 mol % and 30mol %, particularly preferably between 0.1 mol % and 20 mol %, and mostpreferably between 0.5 mol % and 10 mol %, relative to the total amountof unsaturated ethylene monomers. It is particularly preferable that anacrylic acid (salt) serving as an unsaturated ethylene monomer becontained as a main component and the crosslinkable unsaturated monomerbe used in combination with the acrylic acid (salt), because excellentwater absorbing properties are achieved. Note that, instead of thecrosslinkable unsaturated monomer, it is possible to use a compoundhaving two or more unsaturated polymerizable groups other than a vinylgroup and/or two or more reactive functional groups within a molecule.

(2-4) Other Monomer, Other Component

According to the production method 3 of the present invention, a radicalpolymerizable compound may be contained other than the acidradical-containing radical-polymerizable monomer. For example, theradical polymerizable compound other than the acid radical-containingradical-polymerizable monomer is preferably the foregoing unsaturatedethylene monomer (monomer) or a crosslinkable unsaturated monomer(crosslinking agent).

Further, it is possible to use the following radical polymerizablecompound as a compound other than the unsaturated ethylene monomer andthe internal crosslinking agent which are for use in production of thewater absorbing resin and are described in the foregoing (a). That is,it is possible to use a radical polymerizable compound containing anunsaturated ethylene monomer that contains at least one hetero atomother than oxygen, which hetero atom is selected from the groupconsisting of nitrogen, sulfur, phosphorus, silicon and boron.

The amount of the radical polymerizable compound to be used is, but notparticularly limited to, preferably between 0.1 and 20 parts by weight,more preferably between 0.5 and 15 parts by weight, further preferablybetween 1 and 10 parts by weight, particularly preferably between 1.5and 8 parts by weight, and most preferably between 2 and 7 parts byweight, relative to 100 parts by weight of the water absorbing resin.

(3) Polyvalent Metal Compound

A polyvalent metal compound according to the production method 3 of thepresent invention is a compound containing a bivalent or multivalentmetal atom. The polyvalent metal compound is preferably a water-soluble,polyvalent typical metal salt, and further preferably an inorganic ororganic water-soluble aluminum salt. For example, aluminum sulfate(Al₂(SO₄)₃) is a polyvalent metal compound, because the aluminum sulfateis a compound containing trivalent aluminum (Al).

According to the production method 3 of the present invention, usingsuch a polyvalent metal compound provides a water absorbing agentcontaining a polyvalent metal cation(s). This is the same as the waterabsorbing agent production method 2 in which a metallic soap is used. Anoxide of a polyvalent metal (e.g., Al₂O₃), which serves as a polyvalentmetal compound, is not soluble in water, and therefore cannot become acation in water. Therefore, according to the present invention, ahydroxide or a salt, in particular a salt, of a polyvalent metal is usedas a polyvalent metal compound. That is, an inorganic salt or organicsalt, in particular a water-soluble inorganic or organic salt (apolyvalent metal salt of an organic acid), and further a water-solubleinorganic salt (a polyvalent metal salt of an inorganic acid), is used(these are described later).

A polyvalent metal component in the polyvalent metal compound, whichcomponent is usable in the production method 3 of the present invention,preferably contains at least one metal (polyvalent metal cation)selected from typical metals and transition metals of groups 4 to 11.Out of these polyvalent metal components, Mg, Ca, Ti, Zr, V, Cr, Mn, Fe,Co, Ni, Pd, Cu, Zn, Cd, and Al are more preferable. Out of these, Mg,Ca, Zn, and Al are further preferable, and Al is particularlypreferable. It is preferable not to use transition metals, because someof them are polyvalent metal components having relatively high toxicity.

Further, the polyvalent metal compound is preferably soluble in water.According to the present invention, the “water-soluble” means that acompound dissolves, in 100 g of water (25° C.), in an amount of not lessthan 1 g and preferably not less than 10 g.

Examples of the polyvalent metal compound which can be used in thepresent invention include: water-soluble aluminum salts such asaluminium acetate, aluminum lactate, aluminum acrylate, aluminiumchloride, polyaluminium chloride, aluminium sulfate, aluminium nitrate,bis aluminium potassium sulfate, and bis aluminium sodium sulfate;water-soluble alkaline earth metal salts such as calcium chloride,calcium nitrate, magnesium chloride, magnesium sulfate, and magnesiumnitrate; and transition metal salts such as zinc chloride, zinc sulfate,zinc nitrate, copper sulfate, cobalt chloride, zirconium chloride,zirconium sulfate, and zirconium nitrate.

In a case where a polyvalent metal salt of an organic acid is used,examples of a particularly preferable anion include bases correspondingto the following acids: anisic acid, benzoic acid, formic acid, valericacid, citric acid, glycolic acid, glycerol phosphate, glutaric acid,chloroacetic acid, chloropropionic acid, cinnamic acid, succinic acid,acetic acid, tartaric acid, lactic acid, pyruvic acid, fumaric acid,propionic acid, 3-hydroxypropionic acid, malonic acid, maleic acid,butyric acid, isobutyric acid, imidino acetic acid, malic acid,isothionic acid, methyl maleic acid, adipic acid, itaconic acid,crotonic acid, oxalic acid, salicylic acid, gluconic acid, gallic acid,sorbic acid, gluconic acid, and fatty acid, in particular stearic acid,adipic acid and p-hydroxy benzoic acid. Out of these bases, a basecorresponding to tartaric acid and a base corresponding to lactic acidare preferable, and a base corresponding to lactic acid such as aluminumlactate or calcium lactate is most preferable.

Further, in view of solubility, it is preferable to use a salt havingwater of crystallization of those listed above. It is particularlypreferable to use an aluminum compound, that is, a water-solubleinorganic salt of or water-soluble organic salt of aluminum. Out ofthese, aluminum sulfate is preferable, and a powder of a hydrate crystalof for example aluminum sulfate 14-18 hydrate can be used mostpreferably.

Note here that the polyvalent metal compound may remain in the waterabsorbing agent or may change for example through a reaction with awater absorbing resin. Note however that, usually, a resulting waterabsorbing agent will contain almost all the polyvalent metal cation(s)(e.g., aluminum cation) derived from the polyvalent metal compound. Forexample, the amount of the polyvalent metal cation(s) is between 0.001wt % and 5 wt %, and further between 0.005 wt % and 3 wt %. Theseamounts can be easily determined in the method described later.

The amount of the polyvalent metal compound to be used is preferablybetween 0.001 and 20 parts by weight, more preferably between 0.01 and10 parts by weight, and particularly preferably between 0.05 and 5 partsby weight, relative to 100 parts by weight of the solid content of awater absorbing resin composition. Since the amount of the polyvalentmetal compound to be used is not less than 0.001 parts by weight, waterabsorbing properties such as absorbency against pressure are improved.Since the amount of the polyvalent metal compound to be used is not morethan 20 parts by weight, no significant reduction occurs in freeswelling capacity (CRC).

(4) Radical Polymerization Initiator

According to the production method 3 of the present invention, a radicalpolymerization initiator is preferably used in the step (a) of mixingthe water absorbing resin, acid radical-containing radical-polymerizablemonomer, polyvalent metal compound and water. Using a radicalpolymerization initiator cause a decrease in time taken to irradiatewith activating light and the heating time in the step (b). The radicalpolymerization initiator is not particularly limited. Specifically, theradical polymerization initiator is for example a pyrolytic radicalpolymerization initiator or a photodegradable polymerization initiatorfor use in polymerization of the water absorbing resin.

(4-1) Pyrolytic Radical Polymerization Initiator

According to the production method 3 of the present invention, theamount of the pyrolytic radical polymerization initiator to be used isnot particularly limited. Note however that, according to the presentinvention, the amount of the pyrolytic radical polymerization initiatoris preferably between 0.01 and 20 parts by weight, more preferablybetween 0.05 and 10 parts by weight, and further preferably between 0.1and 5 parts by weight, relative to 100 parts by weight of the waterabsorbing resin. Since the amount of the radical polymerizationinitiator falls within the above range, it is possible to obtain a waterabsorbing resin that is excellent in water absorbing properties, andfurther possible to achieve excellent productivity because the reactionspeed is improved in surface treatment. The radical polymerizationinitiator may be used solely or two or more radical polymerizationinitiators may be used in combination.

The pyrolytic radical polymerization initiator is a compound thatgenerates a radical when heated. Out of such compounds, a compoundhaving a ten-hour half-life temperature of between 0° C. and 120° C. ispreferable, a compound having a ten-hour half-life temperature ofbetween 20° C. and 100° C. is more preferable, and a compound having aten-hour half-life temperature of between 40° C. and 80° C. isparticularly preferable. Since the ten-hour half-life temperature is notless than 0° C. (lower limit), the water absorbing agent suffers littledegradation during storage. Since the ten-hour half-life temperature isnot more than 120° C. (upper limit), a reaction proceeds efficiently andthus productivity can be improvised.

The pyrolytic radical polymerization initiator is relatively reasonableas compared to a compound that is sold as a photodegradablepolymerization initiator, and does not necessarily require completelight shielding. Therefore, it is possible to simplify a productionprocess and a production apparatus. Examples of typical pyrolyticradical polymerization initiators include: persulfates such as sodiumpersulfate, ammonium persulfate and potassium persulfate; hydrogenperoxide; and azo compounds such as2,2′-azobis(2-amidinopropane)dihydrochloride,2,2′-azobis[2-2(-imidazoline-2-yl)propane]dihydrochloride, and2,2′-azobis(2-methylpropionitrile). Out of these, persulfates such assodium persulfate, ammonium persulfate and potassium persulfate; and azocompounds such as 2,2′-azobis(2-amidinopropane)dihydrochloride,2,2′-azobis[2-2(-imidazoline 2-yl)propane]dihydrochloride and2,2′-azobis(2-methylpropionitrile), each of which has a ten-hourhalf-life temperature of between 40° C. and 80° C., are preferable. Outof these, it is particularly preferable to use a persulfate, becauseresulting absorbency against pressure, liquid permeability and freeswelling capacity are all excellent. Not only one type of persulfate canbe used, but also two or more persulfates having different counter ionscan be used in combination.

(4-2) Other Polymerization Initiator

According to the production method 3 of the present invention, anoil-soluble photodegradable polymerization initiator and/or anoil-soluble organic peroxide may be used. Examples of the oil-solublephotodegradable polymerization initiator include benzoin derivatives,benzyl derivatives and acetophenone derivatives which are soluble inoil. Examples of the oil-soluble organic peroxide include: ketoneperoxide, peroxyketal, hydroperoxide, dialkyl peroxide, peroxyester andperoxycarbonate which are soluble in oil. The photodegradablepolymerization initiator may be a commercially-available product.Examples of a commercially-available photodegradable polymerizationinitiator include: IRGACURE (registered trademark) 184(hydroxycyclohexyl-phenyl ketone) and IRGACURE (registered trademark)2959(1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one),which are manufactured by Ciba Specialty Chemicals Inc. The amount ofthe photodegradable polymerization initiator to be used is preferablybetween 0.0001 and 1 part by weight, more preferably between 0.001 and0.5 part by weight, and further preferably between 0.005 and 0.1 part byweight, relative to 100 parts by weight of the water absorbing resin.

The radical polymerization initiator for use in the present inventioncan either be oil-soluble or water-soluble. An oil-soluble radicalpolymerization initiator is characterized in that its decompositionspeed is less affected by pH and ionic strength as compared to awater-soluble radical polymerization initiator. Note however that, sincethe water absorbing resin is hydrophilic, it is more preferable to use awater-soluble radical polymerization initiator, in view of permeabilityinto the water absorbing resin. Note that the “water-soluble” means theone that dissolves in water (25° C.) in an amount of not less than 1 wt%, preferably not less than 5 wt %, and more preferably not less than 10wt %.

The water-soluble radical polymerization initiator is for examplepreferably a radical polymerization initiator selected from the groupconsisting of persulfates, hydrogen peroxides and azo compounds.Specific examples of the water-soluble radical polymerization initiatorinclude: persulfates such as ammonium persulfate, sodium persulfate andpotassium persulfate; hydrogen peroxide; and water-soluble azo compoundssuch as 2,2′-azobis-2-amidinopropane dihydrochloride and2,2′-azobis[2-2(-imidazoline-2-yl)propane]dihydrochloride. Out of these,it is particularly preferable to use a persulfate, because absorbencyagainst pressure, liquid permeability and free swelling capacity of theresulting surface-treated water absorbing resin are all excellent.

In addition to the radical polymerization initiator or instead of theradical polymerization initiator, it is possible to further use apercarbonate such as sodium percarbonate; and/or a peracetate such asperacetic acid or sodium peracetate, etc.

According to the present invention, in a case where another radicalpolymerization initiator such as a photodegradable polymerizationinitiator, which is other than the pyrolytic radical polymerizationinitiator, is used as needed in combination with the pyrolytic radicalpolymerization initiator, the amount of the another radicalpolymerization initiator to be used is between 0 and 20 parts by weight,preferably between 0 and 15 parts by weight, and particularly between 0and 10 parts by weight, relative to 100 parts by weight of the waterabsorbing resin. Further, the amount of the another radicalpolymerization initiator to be used is smaller than the amount of thepyrolytic radical polymerization initiator. For example, the amount ofthe another radial polymerization initiator is not more than ½, furthernot more than 1/10, and particularly not more than 1/50 by weight of theamount of the pyrolytic radical polymerization initiator.

[7-B] Production Method of Fourth Embodiment (Water Absorbing AgentProduction Method 3)

(1) Step of Mixing Water Absorbing Resin, Radical Polymerizable Monomer,Polyvalent Metal Compound and Water (Step (a))

According the surface treatment method of the present invention in theproduction method 3 of the present invention, an acid radical-containingradical-polymerizable monomer, a polyvalent metal compound and water aremixed with a water absorbing resin in the step (a).

In the step (a), there is no particular limitation on the order in whichthe radical polymerizable monomer, polyvalent metal compound, water, andif needed a radical polymerization initiator are mixed with the waterabsorbing resin. Accordingly, each of these constituents may beindividually mixed with the water absorbing resin. Alternatively, anaqueous solution containing the radical polymerizable monomer,polyvalent metal compound and radical polymerization initiator may beprepared in advance, and the aqueous solution may be mixed with thewater absorbing resin. Note however that, in order for the radicalpolymerizable monomer etc. to be evenly dispersed on the surface of thewater absorbing resin, it is preferable to prepare an aqueous solutioncontaining the radical polymerizable monomer, polyvalent metal compoundand radical polymerization initiator in advance and then mix the aqueoussolution with the water absorbing resin. Alternatively, the radicalpolymerizable monomer, polyvalent metal compound, radical polymerizationinitiator and the water absorbing resin may be mixed together to obtaina mixture, and thereafter the mixture may be mixed with water.

Note, however, that the scope of the present invention does notencompass the case where the polyvalent metal compound is added to thewater absorbing resin after “the step of polymerizing the acidradical-containing radical-polymerizable monomer (i.e., the step (b),described later)”. In this case, the absorbency against pressure (AAP)is not improved or is reduced.

A solvent in which the radical polymerizable monomer, polyvalent metalcompound and if needed the radical polymerization initiator aredissolved is preferably water alone, although another solvent may becontained provided that their solubility is not lost. That is, it ispreferable to use the radical polymerizable monomer and/or thepolyvalent metal compound and/or the radical polymerization initiator inthe form of an aqueous solution, in the absence of a hydrophobic organicsolvent.

(2) Amount of Water to be Used

The amount of water to be mixed with the water absorbing resin in themixing step (a) is preferably between 1 and 50 parts by weight, morepreferably between 3 and 30 parts by weight, further preferably between5 and 20 parts by weight, and particularly preferably between 6 and 15parts by weight, relative to 100 parts by weight (equivalent to 100 wt %of solid content) of the water absorbing resin. An amount of water to bemixed falling within the above range is preferable, because (i) thereaction speed is improved in surface treatment carried out byirradiation with activating light and/or by heat treatment, (ii) theamount of energy needed is small in the drying step after the treatmentby irradiation with activating light and/or the heat treatment, andfurther (ii) the water absorbing resin is hardly decomposed. In a casewhere the radical polymerizable monomer etc. are to be mixed in the formof an aqueous solution, the amount of water in the aqueous solution canbe controlled so that the amount of water in a resulting water absorbingagent (in particular, the fourth water absorbing agent, further thefirst water absorbing agent) falls within the foregoing range.

It should be noted that how to mix water with the water absorbing resinis not necessarily limited to mixing the water in the form of an aqueoussolution that contains a radical polymerizable monomer etc. For example,water may be mixed after a radical polymerizable monomer, a polyvalentmetal compound, if needed a radical polymerization initiator, and thewater absorbing resin are mixed with each other. Accordingly, a waterabsorbing agent may be obtained by (i) preparing a polymer bypolymerizing a monomer component to obtain a hydrous gel crosslinkedpolymer and thereafter drying the hydrous gel crosslinked polymer untilthe hydrous gel crosslinked polymer has a moisture content that causesthe resultant water absorbing agent to have the moisture content fallingwithin the above range and then (ii) directly mixing a radicalpolymerizable compound, a polyvalent metal compound and if needed aradical polymerization initiator with the polymer.

(3) Mixing Auxiliary Agent

Meanwhile, in order to improve mixability of a water absorbing resincomposition, it is preferable to add a mixing auxiliary agent to thewater absorbing resin composition. Note here that water is notencompassed in the mixing auxiliary agent. When to add the mixingauxiliary agent is not particularly limited. The mixing auxiliary agentis preferably added during or before the step (a). Note here that themixing auxiliary agent other than water is a water-soluble orwater-dispersible compound that is other than radical polymerizablemonomers, polyvalent metal compounds and radical polymerizationinitiators. The mixing auxiliary agent other than water is preferably awater-soluble or water-dispersible compound capable of suppressingaggregation by water of the water absorbing resin and capable ofimproving mixability of the water absorbing resin with an aqueoussolution.

Specifically, surfactants, water-soluble polymers, hydrophilic organicsolvents, water-soluble inorganic compounds, inorganic acids, inorganicsalts, organic acids and organic salts exemplified in Patent Literature30 each can be used as such a water-soluble or water-dispersiblecompound. Adding a mixing auxiliary agent other than water makes itpossible to suppress aggregation by water of the water absorbing resinand thus to evenly mix an aqueous solution and the water absorbingresin. This makes it possible to evenly surface-treat the entire waterabsorbing resin when the water absorbing resin is subjected to treatmentby irradiation with activating light and/or heat treatment in thesubsequent step. It is preferable that a mixing auxiliary agentexemplified in the foregoing water absorbing agent production methods 1and 2 be used as a mixing auxiliary agent also in the production method3.

As described earlier, the amount of a mixing auxiliary agent to be addedis not particularly limited, provided that the mixing auxiliary agent iscapable of suppressing aggregation by water of the water absorbing resinand thus improving mixability of the water absorbing resin with anaqueous solution. For example, the amount of the mixing auxiliary agentto be added is between 0.001 and 40 parts by weight, more preferablybetween 0.01 and 10 parts by weight, and particularly preferably between0.05 and 5 parts by weight, relative to 100 parts by weight of the waterabsorbing resin. Alternatively, according to the present invention, themixing auxiliary agent may be used in an amount of preferably between 0wt % and 40 wt %, more preferably between 0.01 wt % and 20 wt %, andfurther preferably between 0.05 wt % and 10 wt %, relative to the totalamount of an aqueous solution.

In a case where a mixing auxiliary agent exemplified in PatentLiterature 30 etc. is used, there is no particular limitation on how touse the mixing auxiliary agent. The mixing auxiliary agent may be usedin powder form or may be used after being dissolved, dispersed orsuspended in a solution. It is preferable that the mixing auxiliaryagent be used in the form of an aqueous solution.

Further, in the case where the mixing auxiliary agent is used, there isno particular limitation on the order in which the mixing auxiliaryagent is added. The mixing auxiliary agent can be mixed by any methodsuch as (i) a method of adding the mixing auxiliary agent to the waterabsorbing resin in advance to obtain a mixture and thereafter adding anaqueous solution to the mixture or (ii) a method of dissolving themixing auxiliary agent in an aqueous solution and then simultaneouslymixing the mixing auxiliary agent and the aqueous solution with thewater absorbing resin.

(4) Mixing Conditions

In the step (a) in accordance with the present invention, there is noparticular limitation on the mixing conditions in which the waterabsorbing resin, a radial polymerizable monomer, a polyvalent metalcompound, water, and if needed a radical polymerization initiator and amixing auxiliary agent are to be mixed. For example, the mixingtemperature in the step (a) is preferably between 0° C. and 150° C.,more preferably between 10° C. and 120° C., and further more preferablybetween 20° C. and 100° C. Since the mixing temperature is not more than150° C., it is possible to suppress deterioration by heat in the waterabsorbing resin. On the other hand, since the mixing temperature is notless than 0° C., it is possible to carry out stable operations withouttrouble such as condensation of water.

It should be noted that, in a case where the mixing step is carried outat high temperatures, the radical polymerization initiator reacts inresponse to heat even with low doses of irradiation. If this is thecase, it is preferable to prevent excessive leakage of water vapor byfor example sealing a mixing/irradiation system.

Further, the temperature of the water absorbing resin and thetemperature of water before the step (a) are not particularly limited.For example, the temperature of the water absorbing resin before thestep (a) is preferably between 0° C. and 150° C., more preferablybetween 10° C. and 120° C., and further more preferably between 20° C.and 100° C. Since the temperature of the water absorbing resin beforethe step (a) is not more than 150° C., it is possible to suppressdeterioration by heat in the water absorbing resin. On the other hand,since the temperature of the water absorbing resin before the step (a)is not less than 0° C., it is possible to carry out stable operationswithout trouble such as condensation of water.

Further, the temperature of water before the step (a) is preferablybetween 5° C. and 80° C., more preferably between 10° C. and 60° C., andparticularly preferably between 20° C. and 50° C. Since the temperatureof water before the step (a) is not more than 80° C., it is possible tomix a sufficient amount of water with the water absorbing resin bypreventing excessive evaporation of the water before the mixing step(a), and thus possible to bring about the effects of the presentinvention. On the other hand, since the temperature of water is not lessthan 5° C., it is possible to carry out stable operations withouttrouble such as condensation of water.

Further, the mixing time in the step (a) is not particularly limited,provided that those described above can be evenly mixed together.Specifically, the mixing time is preferably between 0.1 second and 60minutes, more preferably between 1 second and 30 minutes, furtherpreferably between 2 seconds and 20 minutes, and most preferably between5 seconds and 10 minutes. If the mixing time is shorter than the lowerlimit, the water absorbing resin, a radical polymerizable monomer, apolyvalent metal compound and water etc. may not be evenly mixed. On theother hand, if the mixing time is longer than the upper limit andbecomes excessive, the water excessively permeates into the waterabsorbing resin. This may inhibit the surface treatment to proceed,which surface treatment is carried out by irradiation with activatinglight and/or by heat treatment.

It should be noted that, when the water absorbing resin, radicalpolymerizable monomer, polyvalent metal compound and water are to bemixed together to obtain a water absorbing resin composition, theseconstituents can be mixed for example with use of an usual mixer such asa V-shaped mixer, a ribbon mixer, a screw-type mixer, a rotating diskmixer, an air mixer, a batch-type kneader, a continuous-type kneader, apaddle-type mixer, or a spade mixer.

(5) Step of Polymerizing Acid Radical-Containing Radical-PolymerizableCompound (Step (b))

The present invention includes the step of polymerizing, by use ofradical generating means such as irradiation with activating lightand/or heating, an acid radical-containing radical-polymerizable monomermixed in the water absorbing resin. It is preferable that the acidradical-containing radical-polymerizable monomer be polymerized in itssurface and/or shallow surface.

The polymerization makes the crosslink density higher in the shallowsurface of the water absorbing resin than inside the water absorbingresin. This achieves excellent properties desired for practical use ofthe water absorbing resin, such as excellent absorbency againstpressure.

The following description discusses two polymerization methods h and i:(5-1) Irradiation with activating light and (5-2) Heating.

(5-1) Irradiation with Activating Light (Polymerization Method h inWater Absorbing Agent Production Method 3)

According to the present invention, irradiation with activating lightmay be carried out (i) while the water absorbing resin, radialpolymerizable monomer, polyvalent metal compound and water are beingmixed or (ii) after two or more of the above are mixed together. Notehowever that, in order to carry out the surface treatment uniformly, itis preferable to (a) obtain a water absorbing resin composition thatcontains the water absorbing resin, radical polymerizable monomer,polyvalent metal compound and water and thereafter (b) irradiate such awater absorbing resin composition with activating light.

The activating light is for example one or more types selected fromultraviolet rays, electron beams and gamma rays, and is preferably anultraviolet ray and/or an electron beam. Taking into consideration aneffect caused by the activating light on human bodies, the activatinglight is more preferably an ultraviolet ray, further preferably anultraviolet ray having a wavelength of not more than 300 nm, andparticularly preferably an ultraviolet ray having a wavelength between180 nm and 290 nm. The irradiation is carried out under the condition inwhich, in a case where an ultraviolet ray is used, the irradiationintensity is preferably between 3 and 1000 [mW/cm²] and the irradianceis preferably between 100 and 10000 [mJ/cm²].

The ultraviolet ray can be emitted with use of for example ahigh-pressure mercury lamp, a low-pressure mercury lamp, a metal halidelamp, a Xenon lamp or a halogen lamp. How the ultraviolet ray is emittedis not particularly limited. Therefore, some other radiation orwavelength may be included provided that a ultraviolet ray, preferably aultraviolet ray having a wavelength of not more than 300 nm, is emitted.It should be noted that, in a case where an electron beam is used, it ispreferable that the acceleration voltage be between 50 kV and 800 kV andthe absorbed dose be between 0.1 Mrad and 100 Mrad.

The irradiation time during which the water absorbing resin isirradiated with activating light depends on the amount of the waterabsorbing resin to be treated. In a case of the method described in thefollowing Examples, the irradiation time may be preferably not less than0.1 minute but less than 60 minutes, more preferably not less than 0.5minute but less than 20 minutes, further preferably not less than 0.5minute but less than 5 minutes, and particularly preferably not lessthan 1 minute but less than 3 minutes. In a case where a conventionalsurface crosslinking agent is used, for example an irradiation time ofmore than 60 minutes is required. That is, the method in accordance withthe present invention makes it possible to shorten the time taken tocarry out the surface treatment, as compared to a conventional method inwhich the crosslink density is the same as in the present invention.

According to the present invention, it is not necessary to carry outheating when carrying out the surface treatment by irradiation withactivating light. Note, however, that the irradiation with activatinglight can be carried out during heating. This makes it possible toobtain a water absorbing resin that is excellent in water absorbingproperties. The heating temperature is preferably between 0° C. and 150°C., more preferably between 10° C. and 120° C., further preferablybetween room temperature and 100° C., and particularly preferablybetween 50° C. and 100° C.

It should be noted that irradiation with activating light may causeradiant heat to be emitted. If this is the case, the irradiation withactivating light is to be carried out during heating. According to thepresent invention, since the surface treatment is carried out byirradiation with activating light, the heating is carried out in anauxiliary manner. Therefore, the temperature for surface treatment canbe set to be lower than that for a conventional surface treatment. Theheating is carried out by for example (i) a method of introducing heatedair into an activating light irradiator, (ii) a method of heating byenclosing the activating light irradiator with a jacket etc., (iii) amethod of heating by utilizing radiant heat emitted when irradiationwith activating light is carried out or (iv) a method of irradiating,with activating light, a water absorbing resin having been heated inadvance.

It is preferable that the water absorbing resin be irradiated withactivating light with stirring. Stirring makes it possible to irradiate,evenly with activating light, a water absorbing resin compositioncontaining the water absorbing resin, radical polymerizable monomer,polyvalent metal compound and water. The water absorbing resin can bestirred, when being irradiated with activating light, with use of forexample a vibration-type mixer, a vibration feeder, a ribbon mixer, aconical ribbon mixer, a screw-type mixing extruder, an air mixer, abatch-type kneader, a continuous-type kneader, a paddle-type mixer, ahigh-speed fluid-type mixer, a floating flow-type mixer or the like.

Alternatively, the water absorbing resin composition may be caused toflow in an apparatus having a shape of a cylinder or a box etc., and theapparatus may be externally irradiated with activating light. In orderto cause the mixture to flow, the pressure of a gas such as air may beutilized like those used in pneumatic transportation of powder. In acase where air is used, it is preferable to humidify the air in order toprevent the water absorbing resin composition from drying. It ispossible to carry out the surface treatment evenly within a short periodof time by irradiating the apparatus with activating light from multipledirections. Note that the apparatus is not particularly limited on itsconstituents and therefore may be constituted by any material, providedthat the material does not inhibit irradiation of the water absorbingresin composition with the activating light. The apparatus isconstituted by for example quartz glass etc.

It has been generally known that a reaction in which a radical acts asan active center is inhibited by oxygen. In this regard, according tothe production method of the present invention, the physical propertiesof the surface-treated water absorbing resin did not decrease eventhough oxygen was present inside the system. This demonstrates that itis not essential that the atmosphere be inert atmosphere duringirradiation with activating light.

Note however that, by causing the atmosphere during the irradiation withactivating light to be inert atmosphere and/or removing dissolved oxygenin agents that are to be mixed with the water absorbing resin in thestep (a), it is possible to shorten the time taken to irradiate withactivating light. This is preferable because this makes it possible toachieve high productivity and low costs. Further, this makes it possibleto reduce the amount of a radical-polymerizable monomer(s) remaining inthe surface-treated water absorbing resin, and thus possible to causethe water absorbing resin to be suitably used in disposable diapers etc.in the field of sanitary materials. Moreover, this provides the sameeffects also in the following (5-2), i.e., in the surface treatment byheating (polymerization method i in water absorbing agent productionmethod 3).

(5-2) Heating (Polymerization Method i in Water Absorbing AgentProduction Method 3)

As described earlier, the acid radical-containing radical-polymerizablemonomer mixed with the water absorbing resin can be polymerized byheating. In a case where the polymerization is carried out by heatingonly, it becomes unnecessary to separately provide an activating lightirradiator. Accordingly, a production apparatus is excellent in design.Further, it becomes possible to improve, at low cost in a safe manner,the water absorbing properties (in particular, absorbency againstpressure) of a resulting surface-treated water absorbing resin.

The mixture obtained in the foregoing step (a) may be prepared under thesame conditions as those for the polymerization by irradiation withactivating light, and a radical polymerization initiator is notessential. It is preferable that an acid radical-containingradical-polymerizable monomer, a polyvalent compound, water and aradical polymerization initiator be mixed with the water absorbing resinin respective predetermined amounts relative to the amount of the waterabsorbing resin. It is preferable that the mixture then be heated at atemperature falling within a predetermined range.

In a case where the step (b) is a step of heating a mixture, the mixtureobtained in the step (a) can be prepared under the conditions differentfrom those for the polymerization by irradiation with activating light.This makes it possible to prevent generation of radicals. The followingdescription discusses a preferred embodiment for heating.

According to the preferred embodiment of the present invention, a waterabsorbing resin is mixed with an aqueous solution containing an acidradical-containing radical-polymerizable monomer, a polyvalent metalcompound and a radical polymerization initiator (hereinafter, such anaqueous solution is also referred to as a “treatment liquid”) to obtaina mixture, i.e., a water absorbing resin composition. Then, the waterabsorbing resin composition is subjected to heat treatment. This seemsto cause a crosslinked structure to be introduced in the surface of thewater absorbing resin. Note, however, that the technical scope of thepresent invention is not limited to such an embodiment. Therefore, thereis no particular limitation on the order in which various componentsexisting in a reaction system are to be added and on timings of additionof the various components and heat treatment. For example, a radicalpolymerizable monomer, a polyvalent metal compound, water and a radicalpolymerization initiator may be separately added to the water absorbingresin (base polymer). Alternatively, these components may be added tothe water absorbing resin (base polymer) during heat treatment.

According to the present invention, there is no particular limitation ona difference between moisture contents before and after the step ofpolymerizing the acid radical-containing radical-polymerizable monomer(hereinafter, such a step is also referred to as a “surface treatmentstep”). Note, however, that it is preferable that the moisture contentafter the surface treatment step be not lower than that before thesurface treatment step, and more preferable that the moisture contentafter the surface treatment step be higher than that before the surfacetreatment step.

(5-2-1) Heating Temperature

The heat treatment is carried out preferably at a temperature of notless than 80° C. but not more than 250° C., more preferably at atemperature between 90° C. and 180° C., and further preferably at atemperature between 100° C. and 150° C. A heating temperature of notless than 80° C. allows the surface treatment to proceed efficiently. Onthe other hand, a heating temperature of not more than 250° C. canprevent deterioration by heat in the water absorbing resin. Heating thewater absorbing resin under the above condition makes it possible toproduce a surface-treated water absorbing resin at low cost in a safemanner, which water absorbing resin is excellent in water absorbingproperties (in particular, absorbency against pressure and liquidpermeability).

When the surface treatment of a water absorbing resin is to be carriedout by heating, the surface treatment may be carried out by heating awater absorbing resin that contains the foregoing components. Anatmosphere in which the heat treatment is carried out is not limited toa particular kind, and it is preferable that heating be carried out inan atmosphere of relatively high humidity. Specifically, it ispreferable to carry out heating in saturated steam or in superheatedsteam. In a case of heating at a temperature of not less than 100° C.,it is preferable that an atmosphere be filled with superheated steam,and more preferable that the water absorbing resin be directly heatedwith use of superheated steam.

(5-2-2) Pressure

The pressure of an atmosphere during the heat treatment is notparticularly limited, and therefore may be reduced pressure, normalpressure or increased pressure. The pressure is preferably between 1013hPa and 43030 hPa, more preferably between 1013 hPa and 14784 hPa,further preferably between 1013 hPa and 10498 hPa, and particularlypreferably between 1013 hPa and 4906 hPa. Further, the relative humidityof the atmosphere is preferably between 50% RH and 100% RH, morepreferably between 70% RH and 100% RH, further preferably between 90% RHand 100% RH, and particularly preferably between 95% RH and 100% RH, andmost preferably 100% RH (saturated steam).

Further, the oxygen concentration in an atmosphere during the heattreatment is preferably between 0% by volume and 25% by volume, morepreferably between 0% by volume between 15% by volume, furtherpreferably between 0% by volume and 10% by volume, further morepreferably between 0% by volume and 5% by volume, particularlypreferably between 0% by volume and 1% by volume, and most preferablybetween 0% by volume and 0.5% by volume. It is preferable that theoxygen concentration in the atmosphere be controlled to relatively lowconcentration like above, because oxidative degradation in the waterabsorbing resin during the heat treatment can be prevented.

(5-2-3) Specific Embodiment of Heating Time and Irradiation withActivating Light

The heating time for the heat treatment is not particularly limitedeither. The heating time is preferably between 1 minute and 90 minutes,more preferably between 2 minutes and 60 minutes, and further preferablybetween 3 minutes and 30 minutes. A heating time of not less than 1minute allows the crosslinked structure to be introduced in the surfaceof the water absorbing resin. On the other hand, a heating time of notmore than 90 minutes can prevent the water absorbing resin fromdeteriorating by heating.

Further, according to the surface treatment step in a surface treatmentmethod in accordance with the present invention, a treatment toirradiate with activating light such as a radiation, an electron beam,an ultraviolet ray, or an electromagnetic beam etc. may be carried outin addition to the foregoing heat treatment.

An apparatus for use in the heat treatment to surface-treat the waterabsorbing resin is not particularly limited, and can be a known dryer.For example, a heat-conduction-type dryer, a heat-radiation-type dryer,a hot-air-heat-conduction-type dryer or a dielectric-heating-type dryeris preferably used. Specifically, a belt-type dryer, an agitated troughdryer, a fluidized bed-type dryer, an air jet dryer, a rotating dryer, akneading dryer, an infrared dryer and an electron beam dryer can bepreferably used.

(6) Other Treatment

Heat treatment may be carried out not only in the polymerization methodi but also in the polymerization method h in the water absorbing agentproduction method 3. That is, after irradiation with activating light,the water absorbing resin may be subjected to heat treatment at atemperature between 50° C. and 250° C. to be dried etc. as needed.

Further, after the irradiation with activating light and/or heating, asurface crosslinkage may be formed with use of a surface crosslinkingagent such as a well-known polyhydric alcohol, polyhydric epoxy compoundor alkylene carbonate.

In addition, an additive may be added in an amount between 0.001 and 20parts by weight, more preferably between 0.01 and 10 parts by weight,and particularly preferably between 0.1 and 5 parts by weight, relativeto 100 parts by weight of the water absorbing resin. Examples of theadditive include: water-insoluble fine particles such as hydrophilicamorphous silica; reducing agents; antimicrobial agents; deodorizers;chelating agents; and polyvalent metal compounds.

[7-C] Particulate Water Absorbing Agent of Fourth Embodiment (FourthWater Absorbing Agent)

According to the present invention, it is possible to produce a novelsurface-treated water absorbing resin by carrying out, with respect to awater absorbing resin, both (i) a surface treatment method includingpolymerizing an acid radical-containing radical-polymerizable monomerand (ii) a surface treatment method using a polyvalent metal compound.The present invention improves not only absorbency against pressure(AAP) but also vertical diffusion absorbency under pressure (VDAUP) of aresulting water absorbing resin. The vertical diffusion absorbency underpressure (VDAUP) is a novel parameter.

That is, according to for example the foregoing production method inwhich polymerization is carried out in the presence of water in anamount between 5 wt % and 20 wt % relative to the amount of the waterabsorbing resin, it is possible to provide a novel water absorbing resinthat is particularly high in water absorbency and also in moisturecontent, in particular a high-moisture-content particulate waterabsorbing agent that is high in vertical diffusion absorbency underpressure (VDAUP).

The fourth water absorbing agent has a VDAUP satisfying the followingcondition. It is preferable that each of the first to fourth waterabsorbing agents also have a VDAUP satisfying the following condition.It is possible to obtain the first and fourth water absorbing agentshaving a novel VDAUP satisfying the following condition by carrying out,in for example the water absorbing agent production methods 1 to 3,control such that a particulate water absorbing agent obtained bypreferably mixing a polyvalent metal compound or a metallic soap (anorganic salt of a polyvalent metal) satisfies the following requirements(1), (2) and (4):

(1) a polyvalent metal cation is contained in an amount between 0.001 wt% and 5 wt % relative to the amount of the particulate water absorbingagent;

(2) an absorbency without pressure (CRC) of the particulate waterabsorbing agent is not less than 28 (g/g) and an absorbency againstpressure (AAP 4.83 kPa) of the particulate water absorbing agent is notless than 10 (g/g); and

(4) a moisture content of the particulate water absorbing agent isbetween 5 wt % and 20 wt %.

The present invention provides a surface-treated polyacrylic acid (salt)water absorbing agent (which has been surface-treated preferably with apolyvalent metal compound, particularly preferably with a water-solublealuminum salt), which water absorbing agent (i) satisfies theinequalities: CRC≧28 [g/g] (preferably CRC≧30 [g/g]), AAP (4.83 kPa)≧10[g/g] (preferably AAP (4.83 kPa)≧15 [g/g]); and VDAUP≧15 g (preferablyVDAUP≧25 g) and (ii) has a moisture content between 5 wt % and 20 wt %.Further, it is preferable that the water absorbing agent satisfy theinequality: AAP (2.07 kPa)≧28 [g/g].

It is preferable that the degree of neutralization of the polyacrylicacid (salt) water absorbing resin be lower in surface than insidethereof. It should be noted that the degree of neutralization inside andin the surface of particles of the water absorbing resin can becalculated by the method described in International Publication No.2009/048160 or the method described in US Patent Application PublicationNo. 2008/0027180.

(Vertical Diffusion Absorbency Under Pressure VDAUP)

According to the present invention, it was found that vertical diffusionabsorbency under pressure (VDAUP), which is a novel parameter indicativeof a physical property, plays an important role in disposable diapers.There have conventionally been proposed, mainly for use in disposablediapers, water absorbing resins in which a lot of physical propertiessuch as absorbency against pressure (AAP), water absorbency (CRC) andliquid permeability (GBP, SFC) etc. are controlled. The inventors of thepresent invention found out the vertical diffusion absorbency underpressure (VDAUP) which is closely correlated to physical properties ofdiapers which properties cannot be evaluated with the above parameters,and developed a particulate water absorbing agent of the presentinvention.

Specifically, according to the fourth water absorbing agent (and furtherthe first to third water absorbing agents), the vertical diffusionabsorbency under pressure VDAUP is preferably not less than 15 g, morepreferably not less than 20 g, further preferably not less than 25 g,further more preferably not less than 30 g, particularly preferably notless than 33 g, and most preferably not less than 35 g. In order to bebalanced with the other physical properties, the upper limit of theVDAUP is not more than 100 g, and further about not more than 80 g. Thevertical diffusion absorbency under pressure is measured by the methoddescribed in Examples.

The vertical diffusion absorbency under pressure (VDAUP) is a parameterbased on a novel idea. It was found that the VDAUP was closelycorrelated to Re-Wet and absolute absorbency in diapers.

The absorbency against pressure (AAP) is defined as a ratio by mass of0.900 g of a water absorbing resin to an absorbed liquid (i.e., definedas water absorbency g/g). On the other hand, the vertical diffusionabsorbency under pressure (VDAUP) is measured in the same manner as inthe method of measuring the AAP except that the amount of an absorbedliquid (absolute amount (g)) is measured with use of 10.000 g of a waterabsorbing resin (i.e., defined as absolute amount (g)).

That is, the VDAUP is measured with use of a water absorbing resinhaving more than 11 times (=10.0/0.9) as much basis weight per unit areaas those used in measuring the AAP. Therefore, interlayer liquiddiffusibility and interlayer liquid permeability in swollen gel layersplay important roles for the liquid to be evenly absorbed in an entiresample. That is, the vertical diffusion absorbency under pressure servesas an index indicative of not only the absorption (g) under pressure persample, but also the liquid diffusibility and liquid permeability in anabsorbent core for practical use, in particular in an absorbent corehaving a high water absorbing resin concentration. The followingExamples also describe the fact that the VDAUP is closely correlated tophysical properties of diapers.

(Moisture Content)

Further, according to a surface-treated water absorbing resin obtainedin the present invention, it is preferable that the moisture content ofthe water absorbing resin after the surface treatment step be higherthan that before the surface treatment step. In particular, it ispreferable that the water absorbing resin be subjected to heat treatmentwith use of saturated steam.

The moisture content of the particulate water absorbing agent of thefourth embodiment (the fourth water absorbing agent) obtained by forexample the water absorbing agent production method 3, i.e., themoisture content of a surface-treated water absorbing resin, fallswithin the range described in the (6-6) of [6].

The moisture content of each of the first, third, and fourth (andfurther the second) particulate water absorbing agents is between 5 wt %and 20 wt %, preferably between 5 wt % and 18 wt %, more preferablybetween 6 wt % and 18 wt %, further preferably between 7 wt % and 15 wt%, and particularly preferably between 8 wt % and 13 wt %. A moisturecontent of less than 5 wt % is not preferable, because sufficientstability to shock is not achieved, and fluidity after moistureabsorption decreases and thus handleability decreases. On the otherhand, a moisture content of more than 20 wt % is not preferable, becausewater absorbing properties (e.g., AAP, CRC) decrease and the fluidityafter moisture absorption decreases.

To what extent the moisture content after the surface treatment step ishigher than that before the surface treatment step is not particularlylimited. The moisture content after the surface treatment step is higherpreferably by 0.1 wt % to 40 wt %, more preferably by 0.5 wt % to 30 wt%, further preferably by 1 wt % to 20 wt %, and particularly preferablyby 2 wt % to 15 wt %, in terms of absolute value of the moisturecontent.

Since the moisture content and an increase in moisture content of theparticulate water absorbing agent are not less than the lower limits ofthe above ranges, the operation and effect of the present invention arefully achieved and the shock resistance (Patent Literatures 17, 20 and32) is sufficiently improved. Unlike Patent Literatures 17, 20 and 32,according to the present invention, it is possible to provide a waterabsorbing resin having a high absorbency against pressure (AAP) and ahigh rate of water absorption (Vortex), and having a predeterminedamount of water (preferably 0.01 wt % to not more than 20 wt %).Accordingly, it is possible to obtain a surface-treated particulatewater absorbing agent which is excellent in water absorbing propertiessuch as absorbency against pressure and liquid permeability. On theother hand, since the moisture content and an increase in moisturecontent are not more than the upper limits of the foregoing ranges, itis possible to prevent a reduction in water absorbing properties, whichreduction is caused by increase in moisture content.

(Residual Monomer)

As shown in Patent Literature 33, surface crosslinking seems to resultin an increase in amount of residual monomers in the fourth waterabsorbing agent (and further the first to third water absorbing agents);however, according to the present invention, it is possible to achieveboth the high physical properties and the small number of residualmonomers, which are in a trade-off relationship. That is, according to asurface-treated water absorbing resin obtained in the present invention,the amount of an acid radical-containing radical-polymerizable compound,which remains particularly in a case where irradiation with activatinglight is carried out, is dramatically reduced. The amount of the acidradical-containing radical-polymerizable compound is reduced topreferably not more than 500 ppm, more preferably not more than 300 ppm,further preferably not more than 200 ppm, and particularly preferablynot more than 100 ppm, relative to the water absorbing resin. Such awater absorbing resin is suitably usable in disposable diapers etc. inthe field of sanitary materials.

(Absorbency Against Pressure AAP)

The AAP 4.83 kPa of the particulate water absorbing agent of the fourthembodiment (the fourth water absorbing agent), i.e., the AAP 4.83 kPa ofa surface-treated water absorbing resin, falls within the rangedescribed in the foregoing (6-1) of [6].

Further, the absorbency against pressure AAP 2.03 kPa of the fourth (andfurther the first to third) water absorbing agent(s) is preferably notless than 25 [g/g], further not less than 28 [g/g], and particularly notless than 30 [g/g]. There is no particular upper limit on the absorbencyagainst pressure AAP 4.83 kPa and the AAP 2.03 kPa. Note however that,in order to be balanced with the other physical properties, the upperlimit is not more than 40 [g/g], and further not more than about 35[g/g]. The absorbency against pressure is measured in the methoddescribed in Examples.

(CRC)

The CRC of the particulate water absorbing agent of the fourthembodiment (the fourth water absorbing agent), i.e., the CRC of asurface-treated water absorbing resin, falls within the range describedin the foregoing (6-3) of [6].

(Particle Size)

The particle size of the particulate water absorbing agent of the fourthembodiment (the fourth water absorbing agent), i.e., the particle sizeof a surface-treated water absorbing resin, falls within the rangedescribed in the foregoing (6-5) of [6].

The powder is obtained for example in the form of spheres, in the formof irregular fragments (obtained by the pulverization step after theaqueous polymerization), in the form of substantial spheres or in theform of granulated (combined) particles. In view of the rate of waterabsorption, the powder is preferably in the form of irregular fragments(obtained by the pulverization step after the aqueous polymerization).

(Other)

A method in accordance with the present invention brings about an effectof granulating fine powders generated during production of a waterabsorbing resin, when surface-treating the water absorbing resin.Accordingly, even if a water absorbing resin that has not yet beensurface-treated contains fine powders, it is possible to obtain asurface-crosslinked water absorbing resin containing less fine powders.This is because the fine powders contained are granulated by the surfacetreatment method in accordance with the present invention.

Further, according to the present invention, it is possible tosufficiently surface-treat the water absorbing resin even at reactiontemperatures around room temperature, depending on conditions. Further,a resulting surface-treated water absorbing resin is very high inproperties desired for the water absorbing resin, such as waterabsorbency, liquid permeability, rate of absorption, gel strength andsuction etc. In addition, the amount of impurities such as residualmonomers is small. Accordingly, a water absorbing resin obtained by thepresent invention is suitably used as a sanitary cotton or a disposablediaper which cause little Re-Wet and little leakage or as a sanitarymaterial which absorbs other bodily fluids.

[8] Absorbent Core, Absorbing Article

A particulate water absorbing agent of the present invention is used forabsorbing water, and is widely used as an absorbent core or an absorbingarticle. The particulate water absorbing agent is particularly suitablyused as a sanitary material for absorbing bodily fluids such as urineand blood. By using the particulate water absorbing agent of the presentinvention in an absorbent core or an absorbing article, the absorbentcore not only (i) fully demonstrates the properties inherent to thewater absorbing agent in the absorbent core by virtue of excellent shockresistance and high absorbing properties (e.g., AAP, CRC) but also (ii)provides excellent effects on the Re-Wet and the rate of absorption ofthe absorbent core by virtue of excellent blendability with tissue suchas pulp.

An absorbent core and an absorbing article of the present invention eachcontain a particulate water absorbing agent of the present invention.

As used herein, the absorbent core means a shaped absorptive materialwhich contains as main components a particulate water absorbing agentand hydrophilic tissue. The content (core concentration) of theparticulate water absorbing agent in the absorbent core relative to thetotal weight of the particulate water absorbing agent and thehydrophilic tissue is between 20 wt % and 100 wt %, more preferablybetween 25 wt % and 90 wt %, particularly preferably between 30 wt % and80 wt %, and most preferably between 40 wt % and 80 wt %. As the coreconcentration in the absorbent core increases, the absorbent core andthe absorbing article etc. become more susceptible to the waterabsorbing properties of the particulate water absorbing agent duringtheir production.

The absorbing article is constituted by the absorbent core, a top sheethaving liquid permeability, and a back sheet having no liquidpermeability. The absorbing article is made by for example (i) producingan absorbent core (core) by blending the particulate water absorbingagent with a fiber base such as hydrophilic tissue or by sandwiching theparticulate water absorbing agent between fiber bases and (ii)sandwiching the absorbent core between a top sheet having liquidpermeability and a back sheet having no liquid permeability. After that,an elastic material, a diffusion layer and/or an adhesive tape etc.are/is provided as needed to obtain an absorbing article such as adiaper for adults or a sanitary napkin. In this case, the absorbent coreis compression-molded to have a density of between 0.06 and 0.50 [g/cm³]and a basis weight of between 0.01 and 0.20 [g/cm³]. Examples of thefiber base to be used include: hydrophilic tissue such as fractured woodpulp; cotton linter and crosslinked cellulose fiber; rayon; cotton;wool; acetate; vinylon and the like. The fiber base is preferably thoseair-laid.

EXAMPLES

The following description more specifically discusses the presentinvention with reference to Examples and Comparative Examples.Electrical appliances etc. used in Examples and Comparative Exampleswere all operated at 100 V and 60 Hz. Further, unless otherwise stated,each physical property was measured at room temperature (25±2° C.) witha relative humidity of 50% RH.

It should be noted that physical properties of a particulate waterabsorbing agent of the present invention are defined by the evaluationmethods described below. Further note that, for convenience ofdescription, the term “parts by weight” may be shortened to “parts” andthe term “liter” may be shortened to “L”.

[Evaluation Method 1] Centrifuge Retention Capacity (CRC)

0.200 g (referred to as weight W0 [g]) of water absorbing resinparticles or particulate water absorbing agent is placed uniformly in abag (85 mm×60 mm) made of nonwoven fabric (manufactured by NANGOKU PULPINDUSTRY CO., LTD, product name: Heatlon Paper Type: GSP-22), and thebag was heat-sealed. After that, the bag was immersed in a large excess(usually, about 500 ml) of 0.90 wt % solution of sodium chloride at roomtemperature. After 30 minutes, the bag was pulled out, and water wasspun off with a centrifugal force (250 G) described in ERT 441.2-02 for3 minutes, with use of a centrifugal separator (manufactured by KOKUSANCo. Ltd., Centrifuge: Type H-122). After that, the weight W1 [g] of thebag was measured.

The same operations were carried out except that neither the waterabsorbing resin particles nor the particulate water absorbing agent wasplaced in the bag, and the weight W2 [g] of the bag was measured. Thecentrifuge retention capacity (CRC) was calculated from the W0 [g], W1[g] and W2 [g] according to the following equation.CRC[g/g]={(W1−W2)/W0}−1  [Math. 1]

[Evaluation Method 2] Absorbency Against Pressure (AAP 4.83 kPa)

The absorbency against pressure (AAP 4.83 kPa) was measured inaccordance with ERT 441.2-02.

0.9 g (referred to as weight W3 [g]) of water absorbing resin particlesor particulate water absorbing agent was placed in a measuringapparatus, and the total weight W4 [g] of the measuring apparatus wasmeasured. Next, a 0.9 wt % solution of sodium chloride was allowed to beabsorbed into the water absorbing resin particles or the particulatewater absorbing agent under a pressure of 4.83 kPa. After 1 hour, thetotal weight W5 [g] of the measuring apparatus was measured. Theabsorbency against pressure (AAP 4.83 kPa) was calculated from the W3[g], W4 [g] and W5 [g] according to the following equation. (Refer toFIG. 2 for the apparatus.)AAP[g/g]=(W5−W4)/W3  [Math. 2]

Absorbency against pressure (AAP 2.07 kPa) was measured in the samemanner, except that the pressure applied on the water absorbing resinparticles or the particulate water absorbing agent was 2.07 kPa.

[Evaluation Method 3] Weight Median Particle Size (D50) and LogarithmicStandard Deviation (σζ) of Particle Size Distribution

The weight median particle size (D50) and the logarithmic standarddeviation (σζ) of particle size distribution were measured in accordancewith the measurement method disclosed in the International PublicationNo. 2004/069404.

Water absorbing resin particles or a particulate water absorbing agentwas classified with use of JIS standard sieves (JIS Z8801-1 (2000))having respective mesh opening sizes of 850 μm, 710 μm, 600 μm, 500 μm,425 μm, 300 μm, 212 μm, 150 μm, 106 μm and 45 μm or sieves equivalent tothe above, and residual percentages R were plotted on logarithmic graphpaper. Then, a particle size satisfying R=50 wt % was read as the weightmedian particle size (D50).

The logarithmic standard deviation (σζ) of particle size distribution isrepresented by the following equation. The smaller the value, thenarrower the particle size distribution. Note that, according to thepresent invention, it is essential to use the foregoing JIS standardsieves to measure and define theσζ=0.5×1n(X2/X1)  [Math. 3]

In the equation, X1 represents a particle size satisfying R=84.1 wt %,and X2 represents a particle size satisfying R=15.9 wt %.

[Evaluation Method 4] Amount of Generation of Coarse Particles(Aggregated Particles)

(i) The amount of particles remaining on a JIS standard sieve 850 μm wasmeasured before and after water was added to a water absorbing resinpowder to obtain a water absorbing agent. An increase [wt %] in theamount of the particles remaining on the JIS standard sieve 850 μm,which amount increased by addition of water, was used as the amount [wt%] of generated coarse particles (aggregated particles).

[Evaluation Method 5] Moisture Content

The moisture content defines the amount of water contained in waterabsorbing resin particles or a particulate water absorbing agent. 1 g ofwater absorbing resin particles or particulate water absorbing agent isdried at 180° C. for 3 hours, and a decrease in the weight of the waterabsorbing resin particles or the particulate water absorbing agent ismeasured. The ratio (wt %) of the weight after drying to the weightbefore drying serves as the moisture content.

The moisture content was measured in the following manner. 1 g (referredto as weight W6 [g]) of water absorbing resin particles or particulatewater absorbing agent was placed in an aluminum cup (whose weight isreferred to as weight W7 [g]) having a bottom surface of about 5 cm indiameter, and was allowed to stand in a calm dryer at 180° C. After 3hours, the total weight (referred to as weight W8 [g]) of the aluminumcup and the water absorbing resin was measured, and the moisture contentwas calculated according to the following equation.Moisture content[wt %]={W6−(W8−W7)}/W6×100  [Math. 4]

[Evaluation Method 6] Paint Shaker Test (PS)

The paint shaker test (PS) is carried out in the following manner. In aglass container which is 6 cm in diameter and 11 cm in height, (i) 10 gof glass beads of 6 mm in diameter and (ii) 30 g of water absorbingresin particles or water absorbing agent are placed, and the glasscontainer is attached to a paint shaker (manufactured by Toyo SeikiSeisaku-sho, Ltd., Product No. 488). Then, the glass container is shakenat 800 [cycle/min] (CPM) for 30 minutes. Details of the apparatus aredisclosed in Japanese Patent Application Publication, Tokukaihei, No.9-235378. After the shaking, the glass beads are removed with use of aJIS standard sieve having a mesh opening size of 2 mm. In this way,damaged water absorbing resin particles or a damaged water absorbingagent are/is obtained.

It should be noted that (i) a paint shaker test in which the shaking iscarried out for 30 minutes may be specifically referred to as a paintshaker test 1 and (ii) a paint shaker test in which the shaking iscarried out for 10 minutes may be specifically referred to as a paintshaker test 2. Note however that, unless otherwise noted, those merelydescribed as a “paint shaker test” indicate the paint shaker test 1 inwhich the shaking is carried out for 30 minutes.

[Evaluation Method 7] Fluidity after Moisture Absorption (Blocking Rateafter Moisture Absorption)

About 2 g of water absorbing resin particles or particulate waterabsorbing agent was spread uniformly in an aluminum cup which is 52 mmin diameter, and was allowed to stand for 1 hour in a thermo-hygrostat(manufactured by TABAI ESPEC CORPORATION, PLATINOUS LUCIFFER PL-2G) inwhich a temperature was 25° C. and a relative humidity was 90±5% RH.After 1 hour, the water absorbing resin particles or the particulatewater absorbing agent in the aluminum cup were/was gently transferredonto a JIS standard sieve (THE IIDA TESTING SIEVE: 80 mm in internaldiameter) having a mesh opening size of 2000 μm (JIS 8.6 mesh), and wasclassified for 5 seconds at room temperature (between 20° C. and 25° C.)with a relative humidity of 50% RH, with use of a Ro-Tap sieve shaker(manufactured by iida-seisakusho Japan Corporation., Sieve shaker typeES-65; Rotation speed: 230 rpm, Impact: 130 rpm). The weight (W9 [g]) ofwater absorbing resin particles or particulate water absorbing agentremaining on the JIS standard sieve and the weight (W10 [g]) of waterabsorbing resin particles or particulate water absorbing agent havingpassed through the JIS standard sieve were measured, and the fluidityafter moisture absorption (blocking rate after moisture absorption) wascalculated according to the following equation. Note that the smallervalue of the fluidity after moisture absorption means more excellentfluidity after moisture absorption.Blocking rate after moisture absorption[wt %]={W10/(W9+W10)}×100  [Math.5]

[Evaluation Method 8] Dusting Rate

A paint shaker test (PS) same as in the evaluation method 6 was carriedout except that the shaking was carried out for 60 minutes. In this way,damage was caused to a particulate water absorbing agent.

The percentage of particles generated during the paint shaker test,which particles have a particle size of not more than 105 μm, wasevaluated as the dusting rate. That is, the lower the dusting rate is,the more excellent is the stability to shock of the water absorbingagent.Dusting rate[wt %]={Amount of particles of not more than 150 μm indiameter after PS}−{Amount of particles of not more than 150 μm indiameter before PS}  [Math. 6]

[Evaluation Method 9] Evaluation of Stirring Torque

The “evaluation of stirring torque” according to the present inventionis evaluation of load applied on a mixer when water or an aqueousdispersion of a metallic soap is added to a water absorbing resin.

Specifically, 50 g of a water absorbing resin was poured into a 600 mLPack Ace container, and thereafter the container was placed in a HAAKEtorque rheometer having four stirring blades. A predetermined amount ofwater or an aqueous dispersion of a metallic soap was added and mixedover about 10 seconds with stirring at 128 rpm, and the stirring torqueduring this operation was outputted to a recorder. After the water orthe aqueous dispersion of the metallic soap was added, the stirringtorque was read at intervals of 2 seconds for 30 seconds. In this way,load applied on the mixer was evaluated.

[Evaluation Method 10] Solid Content

The solid content was calculated using the moisture content found in theevaluation method 5, according to the following equation.Solid content[wt %]=100−Moisture content  [Math. 7]

[Evaluation Method 11] Extractable Content (Ext)

184.3 g of a 0.90 wt % solution of sodium chloride was measured andpoured into a 250 mL plastic container with a lid. To this solution,1.00 g of a particulate water absorbing agent was added, and the mixturewas stirred for 16 hours, thereby extractable content in the particulatewater absorbing agent was extracted. A liquid thus extracted wasfiltered with use of a piece of filter paper (manufactured by AdvantecToyo Kaisha, Ltd.; Product name: JIS P3801, No. 2; thickness: 0.26 mm;diameter of retained particles: 5 μm) to obtain filtrate. 50.0 g of thefiltrate thus obtained was used as a liquid for measurements.

The liquid for measurements was titrated with a 0.1N NaOH solution untilpH 10 was reached, and was then titrated with a 0.1N HCl solution untilpH 2.7 was reached. In this way, titers ([NaOH] mL and [HCl] mL) weremeasured. Further, the same operations were carried out with respect toa 0.90 wt % solution of sodium chloride alone. In this way, blank titers([bNaOH] mL and [bHCl] mL) were measured.

The Ext (extractable content) was calculated from the titers obtained bythe above operations and the average molecular weight of monomers,according to the following equation 8. It should be noted that, in acase where the average molecular weight of monomers was unknown, theaverage molecular weight of monomers was calculated from a degree ofneutralization calculated according to the following equation 9.Ext(wt %)=0.1×(Average molecular weight ofmonomer)×184.3×100×([HCl]−[bHCl])  [Math. 8]Degree of neutralization[mol %]={1−([NaOH]−[bHaOH])/([HCl]−[bHCl])}×100

[Evaluation Method 12] Rate of Water Absorption (Vortex)

Measurement was carried out in accordance with JIS K7224-1996. Note,however, that the temperature of a test liquid (a 0.9 wt % solution ofsodium chloride) was controlled to 30±1° C., and a stirrer chip used was40 mm in length.

[Evaluation Method 13] VDAUP (Vertical Diffusion Absorbency UnderPressure)

The vertical diffusion absorbency under pressure (VDAUP) [g] wascalculated in the same manner as in the AAP, except that the loadapplied on the water absorbing resin was 4.83 kPa, the weight of thewater absorbing resin was 10.000 g, and the calculation was carried outaccording to the following equation.VDAUP[g]=W5−W4  [Math. 10]

[Evaluation Method 14] Residual Monomer Content

The filtrate obtained in the [Evaluation method 11] was analyzed byliquid chromatography using a UV detector. In this way, residual monomercontent (weight ppm relative to water absorbing resin) in the waterabsorbing resin was measured.

[Evaluation Method 15] Evaluation of Performance of Absorbent Core 1

For the purpose of evaluating performance of an absorbent core made fromthe water absorbing resin (or particulate water absorbing agent,described later), the absorbent core was produced and Re-Wet wasevaluated.

(Method of Producing Absorbent Core for Evaluation)

2 parts by mass of a water absorbing resin (or particulate waterabsorbing agent, described later) and 2 parts by mass of fractured woodpulp were dry-blended with use of a mixer. Next, a mixture thus obtainedwas spread on a 400-mesh wire screen (having a mesh opening size of 38μm), and was molded into a web 90 mm in diameter. Further, the web waspressed at a pressure of 196.14 kPa (2 [kgf/cm²]) for 1 minute. In thisway, an absorbent core for evaluation having a basis weight of about0.06 [g/cm²] was obtained.

(Method of Evaluating Re-Wet)

The absorbent core for evaluation was placed on the bottom of a SUS dish90 mm in internal diameter, and a piece of nonwoven fabric 90 mm indiameter was placed on the absorbent core. Subsequently, a piston and aweight were placed on the fabric, which piston and weight have beencontrolled so that a load of 4.8 kPa was evenly applied on the absorbentcore. The piston and the weight used here had a hole for liquidinjection at their center. The hole was 5 mm in diameter. Next, 25 ml ofphysiological saline solution (0.90 wt % solution of sodium chloride)was poured into the center of the absorbent core for evaluation, andallowed to be absorbed into the absorbent core. After 30 minutes,another 25 ml of physiological saline solution (0.90 wt % solution ofsodium chloride) was poured into the center of the absorbent core forevaluation, and was allowed to be absorbed into the absorbent core for30 minutes. After 30 minutes, the piston and the weight, which had beencontrolled so that a load of 4.8 kPa was evenly applied on the absorbentcore, were removed. Then, 30 pieces of filter paper (manufactured byToyo Roshi Kaisha, Ltd., No. 2) each having an external diameter of 90mm, the total weight (W11 [g]) of which was measured in advance, werequickly placed on the absorbent core for evaluation. Further, a pistonand a weight (external diameter: 90 mm, total weight of the piston andthe weight: 20 kg) were quickly placed on the filter paper, which pistonand weight had been controlled so that load was evenly applied on thenonwoven fabric and the filter paper. The load was applied for 5 minutesto allow squeezed-out liquid to be absorbed into the filter paper. Afterthat, the weight (W12 [g]) of the 30 pieces of filter paper wasmeasured, and the Re-Wet after 10 minutes (ten-minute Re-Wet) wascalculated according to the following equation.Ten-minute Re-Wet[g]=W12−W11  [Math. 11]

[Evaluation Method 16] Evaluation of Performance of Absorbent Core 2

(Method of Evaluating Rate of Absorption (Core Acquisition) and Re-Wetof Absorbent Core)

An absorbent core to be evaluated was produced in the following manner.That is, first, 50 parts by weight of an absorbing agent (or waterabsorbing resin) and 50 parts by weight of fractured wood pulp weremoistened with use of an ultrasonic humidifier for 10 seconds, andthereafter were mixed together with use of a mixer. Next, a mixture thusobtained was placed on a 400-mesh wire screen (having a mesh openingsize of 38 μm), and was air-felted to be molded into a web having a sizeof 120 mm×400 mm. The web was pressed at a pressure of 2 kg/cm² (196.14kPa) for 1 minute. In this way, an absorbent core having a basis weightof 0.047 g/cm² was obtained.

Meanwhile, a solution containing 1.9 wt % of urea, 0.8 wt % of NaCl, 0.1wt % of CaCl₂, and 0.1 wt % of MgSO₄ (the rest is water) was prepared.That is, artificial urine was prepared.

A load of 50 g/cm² (4.9 kPa) was applied evenly on the entire absorbentcore, and a cylinder which is 30 mm in diameter and 120 mm in height waspressed against the center of the absorbent core so as to standvertically. Next, 50 g of artificial urine having a temperature of 25°C. was quickly (at one go) poured into the cylinder. A time from whenthe artificial urine started being poured to when the entire artificialurine was absorbed into the absorbent core was measured. The time thusmeasured serves as the first rate of absorption (seconds). After that,with use of the absorbent core used in the above measurement, the samemeasurements were carried out twice at an interval of 50 minutes tomeasure the second rate of absorption (seconds) and the third rate ofabsorption (seconds). 30 minutes after the third artificial urine waspoured, the load was removed from the absorbing article, a paper towel(manufactured by: Oji Paper Co., Ltd., Kitchen Towel Extra Dry, a stackof 30 pieces each having a size of 120 mm×450 mm) was placed on theabsorbing article, and a load of 50 g/cm² (4.9 kPa) was placed thereonand allowed to stand for 1 minute. A change in weight of the paper towelwas measured, thereby the amount of liquid absorbed into the paper towelwas found. The amount thus found was used as the Re-Wet (g).

(Method of Evaluating Blendability with Pulp)

To a mixer containing 50 parts by weight of pulp, 50 parts by weight ofa water absorbing resin (or water absorbing agent) is added while beingmoistened with use of an ultrasonic humidifier. After all of the waterabsorbing resin (or the water absorbing agent) is added, the mixture isstirred for another 1 minute. The blendability of the water absorbingagent (or the water absorbing resin) with pulp during this process wasevaluated as follows.

Very Good: Blendability is good (no water absorbing agent was separatedfrom pulp)

Good: Some water absorbing agent was separated from pulp

Poor: A large amount of water absorbing agent was separated from pulp

Very Poor Aggregates of water absorbing agent were present

[How Examples are Related to Water Absorbing Agents and ProductionMethods of the Present Invention]

The following Examples 1-7, Examples 8-14 and [Table 1] to [Table 4] arespecific examples of the water absorbing agent production methods 1 and2 of the present invention, i.e., specific examples of obtaining thefirst to third (and further the fourth) water absorbing agents of thepresent invention by the water absorbing agent production methods 1 and2.

The following Examples 15-23 and [Table 5] to [Table 7] are specificexamples of the water absorbing agent production methods 1 and 2 of thepresent invention, i.e., specific examples of obtaining the first tothird (and further the fourth) water absorbing agents of the presentinvention by the water absorbing agent production methods 1 and 2.

As will be described, the first to fourth water absorbing agents can beobtained by for example the production methods 1 to 3. Note, however,that methods to obtain the first to fourth water absorbing agents arenot limited to the production methods 1 to 3 described earlier. That is,the second water absorbing agent can be obtained by for example thewater absorbing agent production method 1.

The third water absorbing agent can be obtained by for example the waterabsorbing agent production method 2. Further, the first and fourth waterabsorbing agents can be obtained by for example the water absorbingagent production methods 1 to 3.

(Water Absorbing Agent Production Method 1)

A particulate water absorbing agent production method (the waterabsorbing agent production method 1) of the present invention ischaracterized in that an aqueous dispersion containing a metallic soap(an organic salt of a polyvalent metal) and a dispersion stabilizer ismixed with a water absorbing resin.

(Water Absorbing Agent Production Method 2)

A particulate water absorbing agent production method (the waterabsorbing agent production method 2) of the present invention ischaracterized as (i) including the step of adding a metallic soap (anorganic salt of a polyvalent metal) and water to a water absorbing resinand (ii) controlling the moisture content of the water absorbing agentto between 5 wt % and 20 wt %.

(Water Absorbing Agent Production Method 3)

A particulate water absorbing agent production method (the waterabsorbing agent production method 3) of the present invention is amethod of producing a particulate water absorbing agent, the particulatewater absorbing agent containing a water absorbing resin as a maincomponent, said method including: surface-treating the water absorbingresin by a surface treatment method including the steps of (a) mixing anacid radical-containing radical-polymerizable monomer(s), a polyvalentmetal compound and water with the water absorbing resin and (b)polymerizing the acid radical-containing radical-polymerizablemonomer(s).

Production Example 1

2.8 g (0.025 mol % relative to acrylic acid) of polyethyleneglycoldiacrylate (the average number of moles of ethylene oxide added is 8)was dissolved in 5500 g of solution of sodium acrylate (concentration ofmonomers is 35 wt %) having a degree of neutralization of 75 mol % toform a reaction liquid. The reaction liquid was degassed in a nitrogengas atmosphere for 30 minutes.

Next, the reaction liquid was supplied to a reactor formed by attachinga cover to a double-arm type stainless kneader having a capacity of 10liters and equipped with two sigma type blades and a jacket, and theheadspace in the reactor was replaced with nitrogen while thetemperature of the reaction liquid was kept at 30° C. Subsequently, 2.46g of sodium persulfate and 0.10 g of L-ascorbic acid were separatelyadded to the reaction liquid with stirring. As a result, polymerizationstarted after about 1 minute. Then, a generated hydrous gel crosslinkedpolymer was polymerized at a temperature between 30° C. and 90° C. whilebeing crushed. After 60 minutes after the start of polymerization, thehydrous gel crosslinked polymer was taken out.

The hydrous gel crosslinked polymer thus obtained had been crushed andhad a particle size of about 5 mm. The hydrous gel crosslinked polymerthus crushed was spread on a 50-mesh metal net (having a mesh openingsize of 300 μm), and then hot-air dried at 180° C. for 35 minutes. Inthis way, a dried polymer was obtained

Next, the dried polymer thus obtained was pulverized with use of aroller mill, and was classified with use of JIS standard sieves havingrespective mesh opening sizes of 850 μm and 150 μm. In this way, a waterabsorbing resin powder (a) in the form of irregular fragments, whichpowder has a weight median particle size of 360 μm, was obtained. Thewater absorbing resin powder (a) had a centrifuge retention capacity(CRC) of 49.0 [g/g] and a solid content of 5.1 wt %.

To 100 parts by weight of the water absorbing resin powder (a) thusobtained, a surface-treatment agent composed of 0.02 parts by weight ofdiethylene glycol diglycidyl ether, 1.0 parts by weight of propyleneglycol and 3 parts by weight of water was evenly mixed. After that, themixture was subjected to heat treatment at 100° C. for 45 minutes. Inthis way, surface-crosslinked water absorbing resin particles (a) wereobtained.

After the heat treatment, the water absorbing resin particles (a) weredisintegrated until the particles pass through the JIS standard sievehaving a mesh opening size of 850 μm. Next, the particles were subjectedto a paint shaker test. In this way, surface-crosslinked water absorbingresin particles (A) were obtained. The water absorbing resin particles(A) had a centrifuge retention capacity (CRC) of 39.9 [g/g] and amoisture content of 5.2 wt %.

Example 1

A commercially-available aqueous dispersion of calcium stearate (productname: Afco-Disper C; manufactured by ADEKA CHEMICAL SUPPLY CO., LTD.;solid content: 50 wt %; 1 wt % to 2 wt % of polyoxyethylene tridecylether is contained) was diluted with water. In this way, an aqueousdispersion of calcium stearate having a solid content of 6 wt % wasobtained.

To 100 parts by weight of the water absorbing resin particles (A)obtained in Production Example 1, 5 parts by weight (equivalent to 0.3parts by weight of calcium stearate and 4.7 parts by weight of water) ofthe aqueous dispersion of calcium stearate having a solid content of 6wt % was added with stirring. The mixture was mixed for 1 minute. Thestirring torque after 30 seconds from the start of addition was 0.70[N·m]. FIG. 1 shows how the time from the start of addition is relatedto the stirring torque.

The mixture thus obtained was hardened at 60° C. for 1 hour to obtain ahardened product. The hardened product contained water absorbing resinparticles having a particle size of not less than 850 μm in an amount of0.1 wt % relative to the total amount of the water absorbing agent. Thehardened product was disintegrated until it passed through the JISstandard sieve having a mesh opening size of 850 μm. In this way, aparticulate water absorbing agent (1) was obtained.

Example 2

The same operations as in Example 1 were repeated except that 10 partsby weight (equivalent to 0.3 parts by weight of calcium stearate and 9.7parts by weight of water) of an aqueous dispersion of calcium stearatehaving a solid content of 3 wt % was added instead of 5 parts by weightof the aqueous dispersion of calcium stearate having a solid content of6 wt %. In this way, a particulate water absorbing agent (2) wasobtained.

The stirring torque after 30 seconds from the start of addition was 0.83[N·m]. Further, an obtained hardened product contained water absorbingresin particles having a particle size of not less than 850 μm in anamount of 0.1 wt % relative to the total amount of the water absorbingagent.

Production Example 2

To 100 parts by weight of the water absorbing resin powder (a) obtainedin Production Example 1, a surface-treatment agent composed of a liquidobtained by mixing 0.015 part by weight of ethylene glycol diglycidylether, 1.0 part by weight of propylene glycol and 3.0 parts by weight ofwater was evenly mixed. After that, the mixture was subjected to heattreatment at 80° C. for 60 minutes. Next, the mixture thus heated wasdisintegrated until it passed through the JIS standard sieve having amesh opening size of 850 μm. In this way, surface-crosslinked waterabsorbing resin particles (B) were obtained. The water absorbing resinparticles (B) had a centrifuge retention capacity (CRC) of 43.0 [g/g]and a moisture content of 5.5 wt %.

Example 3

To 100 parts by weight of the water absorbing resin particles (B)obtained in Production Example 2, 15 parts by weight (equivalent to 0.3parts by weight of calcium stearate and 14.7 parts by weight of water)of an aqueous dispersion of calcium stearate having a solid content of 2wt %, which was prepared by diluting with water a commercially-availableproduct (product name: Afco-Disper C), was added with stirring. Then,the mixture was mixed for 1 minute. The stirring torque after 30 secondsfrom the start of addition was 0.93 [N·m]. The mixture thus obtained washardened at 60° C. for 1 hour to obtain a hardened product. The hardenedproduct contained water absorbing resin particles having a particle sizeof not less than 850 μm in an amount of 0.4 wt % relative to the totalamount of the water absorbing agent. The hardened product wasdisintegrated until it passed through the JIS standard sieve having amesh opening size of 850 μm. In this way, a particulate water absorbingagent (3) was obtained.

Example 4

The same operations as in Example 1 were repeated except that 3.3 partsby weight (equivalent to 0.3 parts by weight of calcium stearate and 3.0parts by weight of water) of an aqueous dispersion of calcium stearatehaving a solid content of 10 wt % was added instead of 5 parts by weightof the aqueous dispersion of calcium stearate having a solid content of6 wt %. In this way, a particulate water absorbing agent (4) wasobtained.

The stirring torque after 30 seconds from the start of addition was 0.69[N·m]. Further, an obtained hardened product contained water absorbingresin particles having a particle size of not less than 850 μm in anamount of 0.1 wt % relative to the total amount of the water absorbingagent.

Example 5

The same operations as in Example 1 were repeated except that 10 partsby weight (equivalent to 0.1 parts by weight of calcium stearate and 9.9parts by weight of water) of an aqueous dispersion of calcium stearatehaving a solid content of 1 wt % was added instead of 5 parts by weightof the aqueous dispersion of calcium stearate having a solid content of6 wt %. In this way, a particulate water absorbing agent (5) wasobtained.

The stirring torque after 30 seconds from the start of addition was 1.22[N·m]. Further, an obtained hardened product contained water absorbingresin particles having a particle size of not less than 850 μm in anamount of 0.3 wt % relative to the total amount of the water absorbingagent.

Example 6

100 parts by weight of the water absorbing resin particles (A) obtainedin Production Example 1 and 0.5 parts by weight of zinc stearate powder(manufactured by KANTO CHEMICAL CO., INC) were placed in a Loedige mixer(manufactured by Loedige, type: M5R), and were stirred at 330 rpm for 15seconds to obtain a mixture of the water absorbing resin particles andzinc stearate. To the mixture, 5 parts by weight of water was added withstirring, and the mixture was mixed for 1 minute. Here, the stirringtorque after 30 seconds from the start of addition was 0.47 [N·m]. Themixture thus obtained was hardened at 60° C. for 1 hour to obtain ahardened product. The hardened product contained water absorbing resinparticles having a particle size of not less than 850 μm in an amount of0.4 wt % relative to the total amount of the water absorbing agent. Thehardened product thus obtained was disintegrated until it passed throughthe JIS standard sieve having a mesh opening size of 850 μm. In thisway, a particulate water absorbing agent (6) was obtained.

Example 7

The same operations as in Example 6 were repeated except that the amountof zinc stearate powder was changed to 0.1 part by weight and the amountof water was changed to 10 parts by weight. In this way, a particulatewater absorbing agent (7) was obtained.

The stirring torque after 30 seconds from the start of addition was 0.87[N·m]. Further, an obtained hardened product contained water absorbingresin particles having a particle size of not less than 850 μm in anamount of 4.7 wt % relative to the total amount of the water absorbingagent.

Comparative Example 1

To 100 parts by weight of the water absorbing resin particles (A)obtained in Production Example 1, 5.0 parts by weight of water was addedwith stirring. As a result, a stirrer stopped due to overload after 14seconds from the start of addition. FIG. 1 shows how the time from thestart of addition is related to the stirring torque.

Next, a piece of the mixture thus obtained was hardened at 60° C. for 1hour. In this way, a comparative water absorbing agent (1) was obtained.The hardened product thus obtained contained water absorbing resinparticles having a particle size of not less than 850 μm in an amount of9.1 wt % relative to the total amount of the water absorbing agent.

Comparative Example 2

The same operations as in Example 6 were repeated except that, inaccordance with Patent Literature 19, 0.5 part by weight of hydrophilicsilicon dioxide (product name: Aerosil 200, manufactured by NipponAerosil Co., Ltd.) was used instead of zinc stearate. In this way, acomparative water absorbing agent (2) was obtained.

The stirring torque after 30 seconds from the start of addition was 0.56[N·m]. Further, an obtained hardened product contained water absorbingresin particles having a particle size of not less than 850 μm in anamount of 20.4 wt % relative to the total amount of the water absorbingagent.

Comparative Example 3

The same operations as in Example 6 were repeated except that, inaccordance with Patent Literature 18, 5.0 parts by weight of a 10 wt %solution of sodium alum (manufactured by Wako Pure Chemical Industries,Ltd.) was used instead of zinc stearate. In this way, a comparativewater absorbing agent (3) was obtained.

The stirrer stopped due to overload after 24 seconds from the start ofaddition of the solution of sodium alum. Further, an obtained hardenedproduct contained water absorbing resin particles having a particle sizeof not less than 850 μm in an amount of 56.9 wt % relative to the totalamount of the water absorbing agent.

Comparative Example 4

The same operations as in Example 1 were repeated except that 30 partsby weight (equivalent to 0.1 part by weight of calcium stearate and 29.9parts by weight of water) of an aqueous dispersion of calcium stearatehaving a solid content of 0.33 wt % was added instead of 5 parts byweight of the aqueous dispersion of calcium stearate having a solidcontent of 6 wt %. In this way, a comparative water absorbing agent (4)was obtained.

The stirrer stopped due to overload after 20 seconds from the start ofaddition of the aqueous dispersion of calcium stearate. Further, anobtained hardened product contained water absorbing resin particleshaving a particle size of not less than 850 μm in an amount of 25.1 wt %relative to the total amount of the water absorbing agent.

Production Example 3

To 100 parts by weight of the water absorbing resin powder (a) obtainedin Production Example 1, a surface-treatment agent composed of a liquidobtained by mixing 0.03 parts by weight of ethylene glycol diglycidylether, 0.5 parts by weight of propylene glycol, 0.3 parts by weight of1,4-butanediol, and 3 parts by weight of water was evenly mixed. Afterthat, the mixture was subjected to heat treatment at 200° C. for 40minutes. Next, the particles were subjected to the paint shaker test. Inthis way, surface-crosslinked water absorbing resin particles (C) wereobtained. The water absorbing resin particles (C) had a centrifugeretention capacity (CRC) of 37.4 [g/g] and a moisture content of 2.4 wt%.

Example 8

The same operations as in Example 1 were repeated except that the waterabsorbing resin particles (C) were used instead of the water absorbingresin particles (A). In this way, a particulate water absorbing agent(8) was obtained.

The stirring torque after 30 seconds from the start of addition was 0.65[N·m]. Further, an obtained hardened product contained water absorbingresin particles having a particle size of not less than 850 μm in anamount of 0.1 wt % relative to the total amount of the water absorbingagent.

Example 9

The same operations as in Example 1 were repeated except that an aqueousdispersion of zinc stearate (prepared by diluting with water acommercially-available product (product name: Afco-Disper ZD;manufactured by ADEKA CHEMICAL SUPPLY CO., LTD.; solid content: 42.5 wt%; a surfactant is contained)) was used instead of the aqueousdispersion of calcium stearate. In this way, a particulate waterabsorbing agent (9) was obtained.

The stirring torque after 30 seconds from the start of addition was 0.60[N·m]. Further, an obtained hardened product contained water absorbingresin particles having a particle size of not less than 850 μm in anamount of 0.3 wt % relative to the total amount of the water absorbingagent.

Example 10

20 parts by weight of zinc stearate (manufactured by KANTO CHEMICAL CO.,INC.), 8 parts by weight of sodium polyoxyethylene laurylether sulfate(product name: EMAL 20C, manufactured by Kao Corporation, solid content:25 wt %) and 72 parts by weight of water were mixed to obtain an aqueousdispersion of zinc stearate (a).

To 100 parts by weight of the water absorbing resin particles (C)obtained in Production Example 3, 5.0 parts by weight (equivalent to 1.0parts by weight of zinc stearate, 3.6 parts by weight of water, and 0.4parts by weight of dispersion stabilizer) of the aqueous dispersion ofzinc stearate (a) was added with stirring. Then, the mixture was mixedfor 1 minute. The stirring torque after 30 seconds from the start ofaddition was 0.42 [N·m]. The mixture thus obtained was hardened at 60°C. for 1 hour to obtain a hardened product. The hardened productcontained water absorbing resin particles having a particle size of notless than 850 μm in an amount of 0.1 wt % relative to the total amountof the water absorbing agent. The hardened product thus obtained wasdisintegrated until it passed through the JIS standard sieve having amesh opening size of 850 μm. In this way, a particulate water absorbingagent (10) was obtained.

Example 11

20 parts by weight of zinc stearate (manufactured by KANTO CHEMICAL CO.,INC.), 2 parts by weight of sodium dodecyl sulfate (manufactured byKANTO CHEMICAL CO., INC.) and 78 parts by weight of water were mixed toobtain an aqueous dispersion of zinc stearate (b).

The same operations as in Example 10 were repeated except that theaqueous dispersion of zinc stearate (b) was used as an aqueousdispersion of zinc stearate. In this way, a particulate water absorbingagent (11) was obtained.

The amount of zinc stearate added was substantially 1.0 parts by weight,the amount of water added was substantially 3.9 parts by weight, and theamount of dispersion stabilizer added was substantially 0.1 part byweight, relative to 100 parts by weight of the water absorbing resinparticles (C). The stirring torque after 30 seconds from the start ofaddition was 0.45 [N·m]. Further, an obtained hardened product containedwater absorbing resin particles having a particle size of not less than850 μm in an amount of 0.1 wt % relative to the total amount of thewater absorbing agent.

Comparative Example 5

To 100 parts by weight of the water absorbing resin particles (C)obtained in Production Example 3, 1.4 parts by weight of water was addedwith stirring. Then, the mixture was mixed for 1 minute. The stirringtorque after 30 seconds from the start of addition was 0.84 [N·m]. Themixture thus obtained was hardened at 60° C. for 1 hour to obtain ahardened product. The hardened obtained contained water absorbing resinparticles having a particle size of not less than 850 μm in an amount of2.3 wt % relative to the total amount of the water absorbing agent.

After that, the hardened product was disintegrated until it passedthrough the JIS standard sieve having a mesh opening size of 850 μm.Next, a disintegrated product thus obtained and 0.6 parts by weight ofzinc stearate (manufactured by KANTO CHEMICAL CO., INC.) were placed ina Loedige mixer (manufactured by Loedige, Type: M5R), and were stirredat 330 rpm for 15 seconds. In this way, a comparative water absorbingagent (5) was obtained.

Table 1 shows physical properties of the water absorbing resin particles(A) to (C), the particulate water absorbing agents (1) to (11) and thecomparative water absorbing agents (1) to (5). That is, Table 1 shows acomparison of the water absorbing agent production methods 1 and 2 ofthe present invention and the obtained first to fourth water absorbingagents with conventional techniques.

TABLE 1 Concentration Amount of Amount of of water additive waterEvaluation of Percentage of Type of dispersion (Solid content) addedstirring torque particles 1) CRC additive [wt %] [wt %] [wt %] [N · m][wt %] [g/g] Water absorbing 39.9 resin particles (A) Water absorbing43.0 resin particles (B) Water absorbing 37.4 resin particles (C)Particulate water StCa sus. 6.0 0.3 4.7 0.7  0.1 38.1 absorbing agent(1) Particulate water StCa sus. 3.0 0.3 9.7 0.83 0.1 36.5 absorbingagent (2) Particulate water StCa sus. 2.0 0.3 14.7 0.93 0.4 36.8absorbing agent (3) Particulate water StCa sus. 10.0 0.3 3 0.69 0.1 38.8absorbing agent (4) Particulate water StCa sus. 1.0 0.1 9.9 1.22 0.336.4 absorbing agent (5) Particulate water StZn 0.5 5 0.47 0.4 38.1absorbing agent (6) Particulate water StZn 0.1 10 0.87 4.7 36.4absorbing agent (7) Particulate water StCa sus. 6.0 0.3 4.7 0.65 0.136.5 absorbing agent (8) Particulate water StCa sus. 6.0 0.3 4.7 0.6 0.3 36.5 absorbing agent (9) Particulate water StCa sus. 20 1.0 3.6 0.420.1 36.9 absorbing agent (10) Particulate water StCa sus. 20 1.0 3.90.45 0.1 36.9 absorbing agent (11) Comparative water 5 Stopped after 9.138.1 absorbing agent (1) 14 seconds Comparative water Silica 0.5 5 0.5620.4 38.0 absorbing agent (2) Comparative water Sodium 0.5 4.5 Stoppedafter 56.9 37.7 absorbing agent (3) alum 24 seconds Comparative waterStCa sus. 0.33 0.1 29.9 Stopped after 25.1 32.3 absorbing agent (4) 20seconds Comparative water StZn 0.6 1.4 0.84 2.3 38.0 absorbing agent (5)Blocking Rate AAP Moisture after moisture Dusting 4.83 kPa VDAUP contentabsorption rate AAP + [g/g] [g/g] [wt %] [wt %] [wt %] 1.8CRC Waterabsorbing 14.0 5.2 100 2.4 resin particles (A) Water absorbing 13.8 5.5100 2.3 resin particles (B) Water absorbing 16.6 2.4 100 3.0 resinparticles (C) Particulate water 13.1 9.7 0 0.7 81.8 absorbing agent (1)Particulate water 12.6 13.6 0 0.4 78.3 absorbing agent (2) Particulatewater 12.1 17.2 0 0.4 78.3 absorbing agent (3) Particulate water 13.48.1 0 0.7 83.2 absorbing agent (4) Particulate water 12.6 13.7 5 0.478.1 absorbing agent (5) Particulate water 13.1 9.8 0 0.6 81.7 absorbingagent (6) Particulate water 12.5 13.8 6 0.4 78.1 absorbing agent (7)Particulate water 15.8 22.4 7.2 0 0.8 81.6 absorbing agent (8)Particulate water 15.8 22.3 7.3 0 0.8 81.5 absorbing agent (9)Particulate water 16.0 23.1 6.2 0 0.7 82.4 absorbing agent (10)Particulate water 15.9 23.0 6.3 0 0.7 82.4 absorbing agent (11)Comparative water 13.5 9.8 100 2.9 82.1 absorbing agent (1) Comparativewater 8.0 9.7 0 1.2 76.4 absorbing agent (2) Comparative water 8.5 9.2 01.6 76.4 absorbing agent (3) Comparative water 11.1 25.9 20 0.2 69.2absorbing agent (4) Comparative water 16.4 3.6 0 1.3 84.7 absorbingagent (5) 1) Percentage of particles not less than 850 μm in diameter ina hardened product

It should be noted that the abbreviations in Table 1 represent thefollowing terms:

StCa sus.: Aqueous dispersion (suspension) of calcium stearate

StZn: Zinc stearate powder

StZn sus.: Aqueous dispersion (suspension) of zinc stearate

Example 12

The water absorbing resin particles (A) obtained in Production Example 1were continuously supplied at 120 [kg/h] to a continuous-type high-speedstirring mixer (Turbulizer, manufactured by Hosokawa Micron Corporation)while the aqueous dispersion of calcium stearate having a solid contentof 6 wt % (used in Example 1) was sprayed at 6.0 [kg/h] (the amount ofcalcium stearate added was substantially 0.3 wt % and the amount ofwater added was substantially 4.7 wt %, relative to the amount of thewater absorbing resin particles (A)). In this way, the water absorbingresin particles (A) and the aqueous dispersion of calcium stearatehaving a solid content of 6 wt % were continuously mixed.

The continuous mixing was carried out for 10 hours. As a result, loadapplied on a motor of a mixer apparatus remained constant even after 10hours, and no adhesion was found inside the mixer. Further, the mixturethus obtained was continuously heated and hardened with use of a paddledryer at 60° C. for 30 minutes. Furthermore, classification was carriedout with use of a sieving apparatus to separate particles that passedthrough a 850-μm sieve. Particles that did not pass through the 850-μmsieve were pulverized again with use of a roller granulator, andthereafter were mixed with the particles that passed through the 850-μmsieve. In this way, a sized particulate water absorbing agent (12)consisting only of the particles having passed through the 850-μm sievewas obtained. As to the sieving apparatus and the roller granulator,load applied on their motor was constant even after 10 hours.

Examples 13

The same operations as in Example 12 were repeated except that theaqueous dispersion of calcium stearate having a solid content of 2 wt %(used in Example 3) was added at 18.0 [kg/h] (the amount of calciumstearate added was substantially 0.3 wt % and the amount of water addedwas substantially 14.7 wt %, relative to the amount of the waterabsorbing resin particles (A)) instead of the aqueous dispersion ofcalcium stearate having a solid content of 6 wt %. In this way, aparticulate water absorbing agent (13) was obtained.

Even after 10 hours of continuous mixing, load applied on the motor ofthe mixer remained constant and no adhesion was found inside the mixer.Further, also as to the sieving apparatus and the roller granulator,load applied on their motor was constant even after 10 hours.

Example 14

The water absorbing resin particles (A) obtained in Production Example 1and zinc stearate (manufactured by KANTO CHEMICAL CO., INC.) were pouredinto a continuous Loedige mixer (manufactured by Loedige) at 120 [kg/h]and 0.6 [kg/h], respectively, and were continuously stirred and mixed toobtain a mixture. After that, the mixture was continuously poured into acontinuous-type high-speed stirring mixer (Turbulizer, manufactured byHosokawa Micron Corporation), and water was sprayed at 6.0 [kg/h] (theamount of water added was substantially 5.0 wt % relative to the amountof the water absorbing resin particles (A)). Then, the mixture wascontinuously mixed. Subsequent operations were the same as those as inExample 12. In this way, a particulate water absorbing agent (14) wasobtained.

Even after 10 hours of continuous mixing, load applied on the motor ofthe mixer remained constant and no adhesion was found inside the mixer.Further, also as to the sieving apparatus and the roller granulator,load applied on their motor was constant even after 10 hours.

Comparative Example 5

The same operations as in Example 12 were repeated except that water wasadded at 6.0 [kg/h] (the amount of water added was substantially 5.0 wt% relative to the amount of the water absorbing resin particles (A))instead of the aqueous dispersion of calcium stearate having a solidcontent of 6 wt %. As a result, load applied on the motor of the mixingapparatus started to gradually increase soon after the start of additionof water. After 5 minutes, the motor stopped due to overload. After thestirring mixer was stopped, inside of the mixer was checked. It wasfound that a lot of moistened products were adhered around a stirringrod of the mixing apparatus, and a space between stirring blades and abody was partly blocked with the water absorbing resin.

Comparative Example 6

The same operations as in Example 14 were repeated except thathydrophilic silicon dioxide (product name: Aerosil 200, manufactured byNippon Aerosil Co., Ltd.) was added at 0.6 [kg/h] (the amount ofhydrophilic silicon dioxide added was substantially 0.5 wt % relative tothe amount of the water absorbing resin particles (A)) instead of zincstearate. In this way, a comparative water absorbing agent (6) wasobtained.

Even after 6 hours of continuous mixing, load applied on the motor ofthe mixer remained constant and no adhesion was found inside thestirring mixer. However, as to the sieving apparatus and the rollergranulator, load applied on their motor gradually increased, and thenthe roller granulator stopped due to overload after 6 hours. This wasbecause there were a lot of water absorbing resin particles having aparticle size of not less than 850 μm.

Comparative Example 7

The same operations as in Example 12 were repeated except that the 10 wt% solution of sodium alum used in Comparative Example 3 was added at 6.0[kg/h] instead of the aqueous dispersion of calcium stearate having asolid content of 6 wt %. As a result, load applied on the motor of themixer started to gradually increase soon after the start of addition ofwater. After 21 minutes, the motor stopped due to overload. After thestirring mixer was stopped, inside of the mixing apparatus was checked.It was found that a lot of moistened products were adhered around thestirring rod of the mixing apparatus, and a space between the stirringblades and body was partly blocked with the water absorbing resin.

Table 2 shows physical properties of the particulate water absorbingagents (12) to (14) and the comparative water absorbing agent (6). Itshould be noted in Table 2 that the “AAP+1.8 CRC” means that “Absorbencyagainst pressure (AAP 4.83 kPa)+1.8×Absorbency without pressure (CRC)”.The same applies to the other tables shown as below.

That is, Table 2 shows a comparison of the first water absorbing agentof the present invention obtained by the water absorbing agentproduction methods 1 and 2 of the present invention with a conventionaltechnique.

TABLE 2 AAP Moisture CRC 4.83 kPa content AAP + [g/g] [g/g] [wt %]1.8CRC Water absorbing agent 38.3 13.4 9.2 82 (12) Water absorbing agent36.1 12.6 15.3 78 (13) Water absorbing agent 38.2 13.3 9.3 82 (14)Comparative 38.0 8.0 9.0 76 water absorbing agent (6)

(Regarding Table 1, Table 2 and FIG. 1)

As is clear from Table 1, each of the particulate water absorbing agents(1) to (11) obtained by adding a metallic soap and water to the waterabsorbing resin (A) or (B) and controlling the moisture content tobetween 5 wt % and 15 wt % is excellent in absorbency against pressure(AAP), fluidity after moisture absorption (blocking rate after moistureabsorption) and dusting rate (stability to shock). Since each of theparticulate water absorbing agents (1) to (11) applies only a light loadon a mixing apparatus when water is added (i.e., the stirring torque islow), formation of coarse particles having a particle size of not lessthan 850 μm is suppressed when water is added.

Further, the following is clear from Example 1 and Comparative Examples1, 3 and 4 shown in FIG. 1. That is, according to Example 1, the loadapplied on the mixing apparatus when water is added changes little overtime. In contrast, according to Comparative Examples 1, 3 and 4, theload applied on the mixing apparatus dramatically increases over time.

Further, as shown in Examples 12 to 14, according to the water absorbingagents (12) to (14) which apply only a light load on the mixingapparatus when water is added (i.e., the stirring torque is low) andwhich produce few coarse particles having a particle size of not lessthan 850 μm when water is added, it is possible to stably andefficiently produce a water absorbing agent even after many hours ofcontinuous operation.

As to the comparative water absorbing agent (2) which was obtained bymixing hydrophilic silicon dioxide with the water absorbing resinparticles (A) in accordance with Patent Literature 17, the comparativewater absorbing agent (2) shows a significant decrease (by about 4 g/gto 5 g/g) in absorbency against pressure (AAP). Further, in order toobtain a water absorbing agent having a target particle size, it isnecessary to pulverize coarse particles having a particle size of notless than 850 μm, which coarse particles are generated in a large amountwhen water is added (refer to Comparative Example 6). Since this appliesa heavy load on a pulverizer, the comparative water absorbing agent (2)is not suited for continuous operation. Further, the comparative waterabsorbing agent (2) shows a further decrease in absorbency againstpressure (AAP) due to pulverization.

The same applies to the comparative water absorbing agent (3), which wasobtained by mixing a solution of sodium alum with the water absorbingresin particles (A) in accordance with Patent Literature 16. Thecomparative water absorbing agent (3) shows a significant decrease (byabout 4 g/g to 5 g/g) in absorbency against pressure (AAP), and is notsuited for continuous operation due to a heavy load applied to the mixerwhen mixing is carried out.

It was confirmed that, unlike Patent Literatures 14 to 17, the presentinvention provides a water absorbing agent which (i) has high absorbencyagainst pressure (AAP), (ii) is excellent in stability to shock, (iii)applies only a light load on a mixing apparatus and is capable of beingsized under a light load when water is added, and (iv) is capable ofbeing produced continuously over a long period of time.

(Result of Evaluation of Absorbent Core)

Each of the water absorbing agents obtained in Examples 1, 3, and 10 andComparative Examples 1, 4 and 5 was evaluated for blendability withpulp, Re-Wet of an absorbent core, and rate of absorption (coreacquisition) of the absorbent core. The evaluation and measurement werecarried out in accordance with Evaluation of performance of absorbentcore 2 described in Evaluation method 16. The results are shown in Table3.

TABLE 3 Evaluation of absorbent core (diaper) including water absorbingagent of the present invention Absorbing agent AAP Moisture Absorbentcore CRC 4.83 kPa content Blendability Re-Wet Rate of absorption[Seconds] [g/g] [g/g] [wt %] with pulp [g] First time Second time Thirdtime Particulate water 38.1 13.1 9.7 Very Good 5 32 54 80 absorbingagent (1) Particulate water 36.8 12.1 17.2 Very Good 8 33 72 96absorbing agent (3) Particulate water 36.9 16.0 6.2 Good 6 33 66 88absorbing agent (10) Comparative water 38.1 13.5 9.8 Very poor 12 37 138165 absorbing agent (1) Comparative water 32.3 11.1 25.9 Good 11 36 112129 absorbing agent (4) Comparative water 38.0 16.4 3.6 Poor 10 36 107121 absorbing agent (5)

As is clear from the results shown in Table 3, it was confirmed thatsatisfying the requirements of claim 1, like a water absorbing agentsuch as those obtained in Examples 1, 3 and 10 does, makes it possibleto improve stability to shock and blendability with pulp and thuspossible to improve Re-Wet and rate of absorption of an absorbent core,as compared to the water absorbing agents obtained in ComparativeExamples 1, 4 and 5.

Production Example 4

To a 1-liter polypropylene container which is 80 mm in internal diameterand is enclosed by styrene foam (heat insulator), a solution (A)prepared by mixing 257.6 g of acrylic acid, 0.84 g (0.045 mol % relativeto acrylic acid) of polyethyleneglycol diacrylate (molecular weight:523) and 1.58 g of a 1.0 mass % solution of pentasodiumdiethylenetriamine pentaacetic acid, and a solution (B) prepared bymixing 215.2 g of a 48.5 mass % solution of sodium hydroxide and 210.4 gof ion exchanged water having been controlled to have a temperature of50° C. were poured. Here, the solution (B) was quickly added to thesolution (A) with stirring with a magnetic stirrer in an open system sothat the solution (A) and the solution (B) were mixed. In this way, amonomer solution, whose temperature had risen to about 102° C. due toheat of neutralization and heat of dissolution, was obtained.

After the temperature of the monomer solution thus obtained decreased to95° C., 14.30 g of a 3 mass % solution of sodium persulfate was added,and the mixture was stirred for several seconds. After that, in an opensystem, the mixture was poured into a tray-shaped stainless containerwhich has a bottom surface of about 250 mm×250 mm, inside surface ofwhich is coated with Teflon (registered trademark) and whose surfacetemperature had been raised to 100° C. with use of a hot plate (NEOHOTPLATE H1-1000, manufactured by IUCHI SEIEIDO CO., LTD.). Thetray-shaped stainless container had a bottom surface of 250 mm×250 mm, atop surface of 640 mm×640 mm and a height of 50 mm, had a trapezoidalcross-sectional surface when cut in the middle, and had the top surfaceopened.

Upon pouring of the monomer solution into the tray, polymerization ofthe monomer solution started. The polymerization proceeded while themonomer solution was generating water vapor and expanding and foaming inall directions, and thereafter the monomer solution was shrunk to a sizeof somewhat larger than the bottom surface. Such expansion and shrinkagecompleted within about 1 minute. After being allowed to remain in apolymerization container for 4 minutes, a hydrous polymer was taken out.

The hydrous polymer thus obtained was crushed with use of a meat chopperhaving a dice diameter of 9.5 mm (ROYAL MEAT CHOPPER VR400K,manufactured by Iiduka Kogyo Co., Ltd.) to obtain a crushed hydrouspolymer. The crushing was carried out by providing a gel at about 340g/min and, in the meantime, adding deionized water at 48 g/min. Thecrushed gel contained a non-volatile matter in an amount between 50 mass% and 55 mass %.

A crushed hydrous gel crosslinked polymer thus obtained was spread on a50-mesh metal net, and hot-air dried at 180° C. for 35 minutes to obtaina dried polymer. The dried polymer was pulverized with use of a rollermill, and was further classified with use of the JIS standard sieveshaving respective mesh opening sizes of 850 μm and 150 μm. In this way,a water absorbing resin (D) in the form of irregular fragments, whichhas a weight median particle size of 360 μm, was obtained. The waterabsorbing resin (D) had a centrifuge retention capacity (CRC) of 36.0[g/g] and an extractable content of 12.0 mass %.

To 100 parts by mass of the water absorbing resin (D) thus obtained, asurface-treatment agent composed of a liquid obtained by mixing 0.4parts by mass of 1,4-butanediol, 0.6 parts by mass of propylene glycoland 3.0 parts by mass of pure water was evenly mixed. After that, themixture was subjected to heat treatment at 200° C. for 30 minutes.Further, particles of the mixture were disintegrated until they passedthrough the JIS standard sieve having a mesh opening size of 850 μm.Next, the particles were subjected to the paint shaker test 1. In thisway, surface-crosslinked water absorbing resin particles (D) wereobtained.

Example 15

20 parts by mass of zinc stearate (manufactured by KANTO CHEMICAL CO.,INC), 8 parts by mass of sodium polyoxyethylene laurylether sulfate(product name: EMAL 20C, manufactured by Kao Corporation, solid contentis 25 mass %) and 72 parts by mass of water were mixed to obtain anaqueous dispersion of zinc stearate.

To 100 parts by mass of the water absorbing resin particles (D), 5.0parts by mass of the aqueous dispersion of zinc stearate was evenlymixed with stirring, and the mixture was dried at 60° C. for 1 hour toobtain a dried product. The dried product was disintegrated until itpassed through the JIS standard sieve having a mesh opening size of 850μm. In this way, a water absorbing agent (15) was obtained.

Example 16

20 parts by mass of zinc stearate (manufactured by KANTO CHEMICAL CO.,INC.), 2 parts by mass of sodium dodecyl sulfate (manufactured by KANTOCHEMICAL CO., INC.) and 78 parts by mass of water were mixed to obtainan aqueous dispersion of zinc stearate.

To 100 parts by mass of the water absorbing resin particles (D), 5.0parts by mass of the aqueous dispersion of zinc stearate was evenlymixed with stirring, and the mixture was dried at 60° C. for 1 hour toobtain a dried product. The dried product was disintegrated until itpassed through the JIS standard sieve having a mesh opening size of 850μm. In this way, a water absorbing agent (16) was obtained.

Comparative Example 8

In accordance with Patent Literature 9, 100 parts by mass of the waterabsorbing resin particles (D) and 0.6 parts by mass of zinc stearate(manufactured by KANTO CHEMICAL CO., INC.) were placed in a Loedigemixer (manufactured by Loedige, type: M5R), and were stirred at 330 rpmfor 15 seconds. In this way, a comparative water absorbing agent (8) wasobtained.

Comparative Example 9

In accordance with Patent Literature 9, 100 parts by mass of the waterabsorbing resin particles (D) and 1.0 parts by mass of zinc stearate(manufactured by KANTO CHEMICAL CO., INC.) were placed in a Loedigemixer (manufactured by Loedige, type: M5R), and were stirred at 330 rpmfor 15 seconds. In this way, a comparative water absorbing agent (9) wasobtained.

Comparative Example 10

To 100 parts by mass of the water absorbing resin particles (D), 1.4parts by mass of water was evenly mixed with stirring, and the mixturewas dried at 60° C. for 1 hour to obtain a dried product. The driedproduct was disintegrated until it passed through the JIS standard sievehaving a mesh opening size of 850 μm to obtain a disintegrated product.Next, the disintegrated product and 0.6 pats by mass of zinc stearate(manufactured by KANTO CHEMICAL CO., INC.) were placed in a Loedigemixer (manufactured by Loedige, type: M5R), and were stirred at 330 rpmfor 15 seconds. In this way, a comparative water absorbing agent (10)was obtained.

Comparative Example 11

In accordance with Patent Literatures 1 to 4, the same operations as inComparative Example 8 were repeated except that 0.5 parts by mass ofhydrophilic silicon dioxide (product name: Aerosil 200, manufactured byNippon Aerosil Co., Ltd.) was used instead of zinc stearate. In thisway, a comparative water absorbing agent (11) was obtained.

Comparative Example 12

In accordance with Patent Literatures 1 to 4, the same operations as inComparative Example 8 were repeated except that 1.0 parts by mass ofhydrophilic silicon dioxide (product name: Aerosil 200, manufactured byNippon Aerosil Co., Ltd.) was used instead of zinc stearate. In thisway, a comparative water absorbing agent (12) was obtained.

Table 4 shows physical properties of the water absorbing resin particle(D), the water absorbing agents (15) and (16) and the comparative waterabsorbing agents (8) to (12). That is, Table 4 shows a comparison of thewater absorbing agent production methods 1 and of the present inventionand the obtained first to fourth water absorbing agents of the presentinvention with conventional techniques.

TABLE 4 AAP Blocking rate after 150-pass Moisture CRC 4.83 kPa VDAUPmoisture absorption D50 amount content Vortex [g/g] [g/g] [g/g] [wt %][μm] σζ [wt %] [wt %] [sec] Water absorbing 31.9 26.4 100 359 0.37 2.042.4 58.1 resin particles (D) Water 30.4 23.9 30.2 0 360 0.37 2.18 6.260.8 absorbing agent (15) Water 30.5 24 29.9 0 363 0.37 2.2 6.3 60.4absorbing agent (16) Comparative water 31.7 25.1 0.2 362 0.37 1.99 2.468.2 absorbing agent (8) Comparative water 31.8 25.4 0 358 0.37 1.91 2.468.2 absorbing agent (9) Comparative water 31.7 25.2 0 380 0.34 0.81 3.667.3 absorbing agent (10) Comparative water 32.4 22.6 381 0.34 0.93 2.4absorbing agent (11) Comparative water 32.2 21.8 405 0.35 0.97 2.4absorbing agent (12)

(Closing)

As is clear from Table 4, each of the water absorbing agents (15) and(16) obtained by mixing an aqueous dispersion containing a metallic soapand a dispersion stabilizer to the water absorbing resin particles (D)is excellent in absorbency against pressure (AAP) and fluidity aftermoisture absorption (blocking resistance) and has excellent rate ofwater absorption (Vortex).

As to the comparative water absorbing agents (8) and (9) obtained bymixing a metallic soap to the water absorbing resin particles (D) inaccordance with Patent Literature 9, each of the comparative waterabsorbing agents (8) and (9) shows a significant decrease in rate ofwater absorption (Vortex) (i.e., it took about 10 more seconds). Thesame applies to the comparative water absorbing agent (10) obtained bymixing each separately a metallic soap and water. The comparative waterabsorbing agent (10) shows a significant decrease in rate of waterabsorption (Vortex) (i.e., it took about 10 more seconds).

Further, as to the comparative water absorbing agents (11) and (12)obtained by mixing hydrophilic silicon dioxide with the water absorbingresin particles (D) in accordance with Patent Literatures 1 to 4, eachof the comparative water absorbing agents (11) and (12) shows asignificant decrease (by about 4 g/g to 5 g/g) in absorbency againstpressure (AAP). Unlike Patent Literatures 14 to 16, the presentinvention is capable of providing a water absorbing agent which is highin absorbency against pressure (AAP) and rate of water absorption(Vortex) and contains a predetermined amount of water (preferablybetween 0.01 wt % and not more than 20 wt %).

The foregoing description discussed Examples 1 to 16 of the waterabsorbing agent production methods 1 and 2. On the other hand, thefollowing description discusses Examples 17 to 22 which are specificexamples of the water absorbing agent production method 3.

(Water Absorbing Agent Production Method 3)

A particulate water absorbing agent production method (the waterabsorbing agent production method 3) of the present invention is amethod of producing a particulate water absorbing agent, the particulatewater absorbing agent containing a water absorbing resin as a maincomponent, said method including: surface-treating the water absorbingresin by a surface treatment method including the steps of (a) mixing anacid radical-containing radical-polymerizable monomer(s), a polyvalentmetal compound and water with the water absorbing resin and (b)polymerizing the acid radical-containing radical-polymerizablemonomer(s).

Production Example 5

A solution of acrylic acid monomers (concentration of monomers: 38 wt %,degree of neutralization: 75 mol %) composed of sodium acrylate, acrylicacid and water was prepared and placed in a kneader equipped with twosigma type blades. Then, polyethyleneglycol diacrylate (the averagenumber of ethylene oxide units: n=9) serving as an internal crosslinkingagent was dissolved so that the amount of the polyethyleneglycoldiacrylate was 0.023 mol % relative to the amount of the monomers.

Next, a nitrogen gas was blown into the solution to reduce oxygenconcentration in the solution and to replace the headspace in a reactioncontainer by nitrogen. Subsequently, 0.05 mol % (relative to themonomers) of sodium persulfate and 0.0006 mol % (relative to themonomers) of L-ascorbic acid, which serve as polymerization initiators,were added while the two sigma type blades were rotated. The mixture wasstirred and polymerized in the kneader. After 40 minutes, a hydrous gelpolymer having a median particle size of 2 mm was obtained.

The hydrous gel polymer thus obtained was dried for 45 minutes in a hotair dryer in which a temperature was set to 170° C. After that, thedried polymer was pulverized with use of a roller mill pulverizer. Thedried polymer was classified with use of a sieve having a mesh openingsize of 500 μm so that particles having a particle size of more than 500μm were removed, and classified with use of a sieve having a meshopening size of 125 μm so that particles having a particle size of lessthan 125 μm were removed. In this way, a water absorbing resin (E)serving as a base polymer was obtained.

Table 5 shows particle size distribution of the water absorbing resin(E) serving as a base polymer thus obtained. Table 6 shows the resultsof evaluations of the water absorbing resin (E) serving as a basepolymer.

Production Example 6

The same operations as in Production Example 5 were repeated except thatthe step of removing small particles by classification was omitted. Inthis way, a water absorbing resin (F) serving as a base polymer wasobtained.

Table 5 shows particle size distribution of the water absorbing resin(F) serving as a base polymer thus obtained. Table 7 shows the resultsof evaluations of the water absorbing resin (F) serving as a basepolymer thus obtained.

Production Example 7

The same operations as in Production Example 5 were repeated except thatthe sieve for removing large particles was changed to a sieve having amesh opening size of 850 μm and the sieve for removing small particleswas changed to a sieve having a mesh opening size of 180 μm. In thisway, a water absorbing resin (G) serving as a base polymer was obtained.Table 5 shows particle size distribution of the water absorbing resin(G) serving as a base polymer thus obtained. Table 7 shows the resultsof evaluations of the water absorbing resin (G) serving as a basepolymer thus obtained. Note that all the particles were in the form ofirregular fragments.

TABLE 5 Production Example 5 6 7 Water absorbing E F G resin D50 (μm)250 178 474 Particle size distribution 850 μm or greater 0 0 0 (wt %)850 μm to 500 μm 0 0 45.1 (wt %) 500 μm to 300 μm 33.3 23.5 40.0 (wt %)300 μm to 150 μm 55.4 40.2 14.2 (wt %) 150 μm or less (wt %) 11.3 36.30.7 Total (wt %) 100 100 100

It should be noted in Table 5 that the (A μm or greater) means aparticulate water absorbing resin remaining on a sieve having a meshopening size of A μm after the classification operation. The (B μm orless) means a particulate water absorbing resin having passed through asieve having a mesh opening size of B μm. The (C μm to D μm) means aparticulate water absorbing resin which has passed through a sievehaving a mesh opening size of C μm but remained on a sieve having a meshopening size of D μm.

Example 17

30 g of the water absorbing resin (E) serving as a base polymer wasadded in a 500 mL separable flask made of quartz. Then, a solutionprepared in advance by mixing 0.033 g of glycerin dimethacrylate(manufactured by SHIN-NAKAMURA CHEMICAL CO., LTD., NK Ester 701), 0.90 gof acrylic acid, 2.10 g of water, 0.003 g of IRGACURE (registeredtrademark) 2959 (manufactured by Ciba Specialty Chemicals Inc.) and 0.09g of aluminum sulfate 14-18 hydrate was added with stirring by use of astirring blade. After 10 minutes of stirring, the mixture was irradiatedwith UV light at room temperature for a total of 1.5 minutes at 65[mW/cm²] (measured in a position, on a wall of the separable flask madeof quartz, which is nearest to a UV lamp, with use of a uviometerUIT-150 and a light receiving part UVD-S254 manufactured by USHIO INC.),with use of a UV light irradiator (UV-152/1MNSC3-AA06, manufactured byUSHIO INC.) to which a metal halide lamp (UVL-1500M2-N1, manufactured byUSHIO INC.) was attached. In this way, a surface-crosslinked waterabsorbing resin serving as a particulate water absorbing agent (17) wasobtained.

Table 6 shows the results of evaluations of the particulate waterabsorbing agent (17) thus obtained. It should be noted that, in thepresent example and subsequent Examples and Comparative Examples, theevaluation of an absorbent core was carried out in accordance withEvaluation of performance of absorbent core 1 of Evaluation method 15.

Example 18

The same operations as in Example 17 were repeated except that theamount of acrylic acid used was 1.20 g. In this way, asurface-crosslinked water absorbing resin serving as a particulate waterabsorbing agent (18) was obtained. Table 6 shows the results ofevaluations of the particulate water absorbing agent (18) thus obtained.Table 8 shows the results obtained by evaluating performance of anabsorbent core including the particulate water absorbing agent (18).

Example 19

The same operations as in Example 17 were repeated except that theamount of acrylic acid used was 1.50 g. In this way, asurface-crosslinked water absorbing resin serving as a particulate waterabsorbing agent (19) was obtained. Table 6 shows the results ofevaluations of the particulate water absorbing agent (19) thus obtained.

Example 20

The same operations as in Example 17 were repeated except that theamount of aluminum sulfate 14-18 hydrate used was 0.15 g. In this way, asurface-crosslinked water absorbing resin serving as a particulate waterabsorbing agent (20) was obtained. Table 6 shows the results ofevaluations of the particulate water absorbing agent (20) thus obtained.

Example 21

The same operations as in Example 18 were repeated except that 30 g ofthe water absorbing resin (F) serving as a base polymer was used and theamount of aluminum sulfate 14-18 hydrate used was 0.30 g. In this way, asurface-crosslinked water absorbing resin serving as a particulate waterabsorbing agent (21) was obtained. Table 7 shows the results ofevaluations of the particulate water absorbing agent (21) thus obtained.

Example 22

The same operations as in Example 21 were repeated except that theamount of water in the solution was 4.20 g. In this way, asurface-crosslinked water absorbing resin serving as a particulate waterabsorbing agent (22) was obtained. Table 7 shows the results ofevaluations of the particulate water absorbing agent (22) thus obtained.

Example 23

The same operations as in Example 18 were repeated except that 30 g ofthe water absorbing resin (C) serving as a base polymer was used. Inthis way, a surface-crosslinked water absorbing resin serving as aparticulate water absorbing agent (23) was obtained. Table 7 shows theresults of evaluations of the particulate water absorbing agent (23)thus obtained.

Comparative Example 13

The same operations as in Example 17 were repeated except that thealuminum sulfate 14-18 hydrate was not contained in the solution. Inthis way, a comparative water absorbing agent (13) was obtained. Table 6shows the results of evaluations of the comparative water absorbingagent (13) thus obtained.

Comparative Example 14

The same operations as in Example 17 were repeated except thatpolyethyleneglycol monomethyl ether (manufactured by Aldrich,CH₃(OCH₂CH₂)_(n)OH, the number average molecular weight Mn: 2000) wasused in stead of the aluminum sulfate 14-18 hydrate in the solution. Inthis way, a comparative water absorbing agent (14) was obtained. Table 6shows the results of evaluations of the comparative water absorbingagent (14) thus obtained.

Comparative Example 15

The same operations as in Comparative Example 14 were repeated exceptthat the amount of acrylic acid used was 1.20 g. In this way, acomparative water absorbing agent (15) was obtained. Table 6 shows theresults of evaluations of the comparative water absorbing agent (15)thus obtained. Table 8 shows the results obtained by evaluatingperformance of an absorbent core including the comparative waterabsorbing agent (15).

Comparative Example 16

To 30 g of the comparative water absorbing agent (13) obtained inComparative Example 13, 0.39 g of a solution obtained by mixing a 50 wt% solution of aluminum sulfate 14-18 hydrate and a 60 wt % solution ofsodium lactate in proportions of 10:3 by weight was added. In this way,a comparative water absorbing agent (16) was obtained. Table 6 shows theresults of evaluations of the comparative water absorbing agent (16)thus obtained.

Comparative Example 17

The same operations as in Comparative Example 15 were repeated exceptthat 30 g of the water absorbing resin (F) was used as a base polymer.In this way, a comparative water absorbing agent (17) was obtained.Table 7 shows the results of evaluations of the comparative waterabsorbing agent (17) thus obtained.

Comparative Example 18

The same operations as in Comparative Example 17 were repeated exceptthat the amount of water in the solution was 4.20 g. In this way, acomparative water absorbing agent (18) was obtained. Table 7 shows theresults of evaluations of the comparative water absorbing agent (18)thus obtained.

Note that all the particles of the water absorbing agents shown inTables 6 to 8 were in the form of irregular fragments.

TABLE 6 Treatment agent IRGA- Moisture 701 AA W CURE ASH PEGOMe contentWater absorbing resin [wt %] [wt %] [wt %] [wt %] [wt %] [wt %] [wt %]Production Water absorbing 6.9 Example 5 resin (E) Example 17Particulate water 0.11 3 7 0.01 0.3 12.4 absorbing agent (17) Example 18Particulate water 0.11 4 7 0.01 0.3 11.0 absorbing agent (18) Example 19Particulate water 0.11 5 7 0.01 0.3 11.8 absorbing agent (19) Example 20Particulate water 0.11 3 7 0.01 0.5 13.1 absorbing agent (20)Comparative Comparative water 0.11 3 7 0.01 12.8 Example 13 absorbingagent (13) Comparative Comparative water 0.11 3 7 0.01 0.01 12.8 Example14 absorbing agent (14) Comparative Comparative water 0.11 4 7 0.01 0.0112.3 Example 15 absorbing agent (15) Comparative Comparative water 0.113 7 0.01 13.1 Example 16 absorbing agent (16) Physical properties AAPAAP Res. CRC 2.07 kPa 4.83 kPa VDAUP M AAP + [g/g] [g/g] [g/g] [g] [ppm]1.8CRC Production 54.7 Example 5 Example 17 35.5 29.0 17.0 81 Example 1834.0 30.7 21.1 33.4 312 82 Example 19 32.6 30.9 22.5 81 Example 20 35.828.8 16.9 81 Comparative 36.0 26.9 12.0 Example 13 Comparative 37.0 25.710.0 Example 14 Comparative 35.2 27.9 12.9 12.1 2378 Example 15Comparative 35.2 25.2 10.8 Example 16

TABLE 7 First or fourth water absorbing agent obtained by waterabsorbing agent production method 3 Treatment agent IRGA- Moisture 701AA W CURE ASH PEGOMe content Water absorbing resin [wt %] [wt %] [wt %][wt %] [wt %] [wt %] [wt %] Production Water absorbing 7.2 Example 6resin (F) Production Water absorbing 5.3 Example 7 resin (G) Example 21Particulate water 0.11 4 7 0.01 1 14.2 absorbing agent (21) Example 22Particulate water 0.11 4 14 0.01 1 19.1 absorbing agent (22) Example 23Particulate water 0.11 4 7 0.01 0.3 12.4 absorbing agent (23)Comparative Comparative water 0.11 4 7 0.01 0.01 11.3 Example 17absorbing agent (17) Comparative Comparative water 0.11 4 14 0.01 0.0114.1 Example 18 absorbing agent (18) Physical properties AAP AAP Res.CRC 2.07 kPa 4.83 kPa VDAUP M AAP + [g/g] [g/g] [g/g] [g] [ppm] 1.8CRCProduction 50.7 Example 6 Production 52.6 Example 7 Example 21 35.3 16.5Example 22 29.5 22.0 Example 23 33.0 29.0 22.0 52.4 332 81 Comparative44.2 6.5 Example 17 Comparative 41.6 8.3 Example 18

It should be noted that the abbreviations in Tables 6 and 7 representthe following terms:

701: NK Ester 701 (manufactured by SHIN-NAKAMURA CHEMICAL CO., LTD.,glycerin dimethacrylate)

AA: Acrylic acid

W: Pure water

IRGACURE: IRGACURE 2959 (manufactured by Ciba Specialty Chemicals Inc.)

ASH: Aluminum sulfate 14-18 hydrate

PEGOMe: Polyethyleneglycol monomethyl ether (manufactured by Aldrich,CH₃(OCH₂CH₂)_(n)OH, the number average molecular weight Mn: 2000)

Res.M: Residual monomer content

TABLE 8 Evaluation of absorbent core (diaper) including first or fourthwater absorbing agent obtained by water absorbing agent productionmethod 3 Water absorbing agent Re-Wet (g) Example 18 Particulate water 5absorbing agent (18) Comparative Comparative water 11 Example (15)absorbing agent (15)

INDUSTRIAL APPLICABILITY

A water absorbing agent production method in accordance with the presentinvention is suitable for producing a water absorbing agent containingas a main component a water absorbing resin having excellent physicalproperties. A water absorbing agent of the present invention is suitablefor use in sanitary materials such as disposable diapers, sanitarynapkins and incontinence pads.

REFERENCE SIGNS LIST

-   100 Supporting cylinder made of plastic-   101 400-mesh metal net made of stainless steel-   102 Swollen gel-   103 Piston-   104 Load (weight)-   105 Petri dish-   106 Glass filter-   107 Filter paper-   108 0.90 wt % solution of sodium chloride

The invention claimed is:
 1. A particulate water absorbing agentcomprising: a water absorbing resin as a main component, wherein thewater absorbing resin comprises a polyacrylic acid (salt) crosslinkedpolymer; a polyvalent metal, wherein the polyvalent metal is a metallicsoap and/or a water-soluble polyvalent metal salt; and a dispersionstabilizer; wherein said particulate water absorbing agent has: (1) apolyvalent metal cation in an amount between 0.001 wt % and 5 wt %relative to the amount of the particulate water absorbing agent; (2) anabsorbency without pressure (CRC) of not less than 28 (g/g) and not morethan 60 (g/g), and an absorbency against pressure (AAP 4.83 kPa) of notless than 10 (g/g) and not more than 40 (g/g); (3) the absorbencyagainst pressure and the absorbency without pressure satisfy theinequality: 77≦Absorbency against pressure (AAP 4.83 kPa)+1.8×Absorbencywithout pressure (CRC)≦100; (4) a moisture content between 5 wt % and 20wt %; and (5) a dusting rate between 0 wt % and 0.8 wt %.
 2. Theparticulate water absorbing agent according to claim 1, wherein avertical diffusion absorbency under pressure (VDAUP) is not less than 15g.
 3. The particulate water absorbing agent according to claim 1, whichsatisfies any one of the following (A) through (C): (A) the particulatewater absorbing agent is obtained by mixing an aqueous dispersioncontaining a metallic soap and a dispersion stabilizer with the waterabsorbing resin; (B) the particulate water absorbing agent is obtainedby a method including the steps of adding a metallic soap and water tothe water absorbing resin and controlling a moisture content of thewater absorbing agent to between 5 wt % and 20 wt %; and (C) theparticulate water absorbing agent is obtained by carrying out a surfacetreatment method for water absorbing resin, the method including thesteps of (i) mixing an acid radical-containing radical-polymerizablemonomer, a polyvalent metal compound and water with the water absorbingresin which is a polyacrylic acid (salt) crosslinked polymer and (ii)polymerizing the acid radical-containing radical-polymerizable monomer.4. The particulate water absorbing agent according to claim 1, wherein:the metallic soap is contained in an amount between 0.001 and 5 parts byweight relative to 100 parts by weight of the water absorbing resin; andthe dispersion stabilizer is contained in an amount between 0.0001 and 1part by weight relative to 100 parts by weight of the water absorbingresin.
 5. The particulate water absorbing agent according to claim 1,wherein the dispersion stabilizer is a surfactant.
 6. The particulatewater absorbing agent according to claim 1, wherein the water absorbingresin is a surface-crosslinked water absorbing resin.
 7. The particulatewater absorbing agent according to claim 1, wherein a blocking rateafter moisture absorption is between 0 wt % and 10 wt %.
 8. Theparticulate water absorbing agent according to claim 1, which isobtained by granulating water absorbing resin particles.
 9. An absorbingarticle comprising a particulate water absorbing agent recited inclaim
 1. 10. The particulate water absorbing agent according to claim 1,wherein: the absorbency against pressure and the absorbency withoutpressure satisfy the inequality: 77≦Absorbency against pressure (AAP4.83 kPa)+1.8×Absorbency without pressure (CRC)≦90.